All season, the top four teams in the NBA's Eastern Conference seemed to be on a high-stakes playoff collision course. Now those long-awaited postseason meetings have finally arrived, and after four total games in the series between the Boston Celtics and Milwaukee Bucks and the Toronto Raptors and Philadelphia 76ers, chaos appears on the horizon.

How the tied series play out as Celtics-Bucks shifts to Boston and Raptors-Sixers goes to Philadelphia will have significant ramifications not only for each of the teams involved, but also for the rest of the league.

Here's a team-by-team look at what's at stake:

Philadelphia 76ers | 31 percent chance to advance

Biggest things at stake if they lose

1. Philly's title window

"We believe we are in position to contend now, and our moves reflect that belief."

That was what Sixers general manager Elton Brand said back in February after reshaping his team's roster for a second time in just a few months on the job. It also was a clear signal of intent for Philadelphia's immediate expectations.

Owner Josh Harris doubled down on that belief when he declared to ESPN's Jackie MacMullan, "I think we have enough talent to win [it all]. We want to make sure, at a minimum, to advance deeper in the playoffs than we did last year."

So what happens to the Sixers now?

They'll have to spend big bucks to keep Tobias Harris and Jimmy Butler. Ditto for JJ Redick, who signed a second consecutive one-year deal and will be a sought-after free agent despite being 34. Then there is Joel Embiid, whose health remains a long-term question mark, no matter how much he gets frustrated by that line of thinking.

Embiid is good enough that having a healthy version of him can alone keep the Sixers in contention. But to truly remain among the East's elite, they are going to need to retain at least two of Harris, Butler and Redick -- if not all three -- while making additions to the bench, which has been a major weakness after big moves this season.

2. How much it will cost to retain Harris and Butler

Deciding what Harris is going to get will be easy. Several teams are almost certain to come after him with four-year max offers, given that the combo forward is an excellent shooter who can run pick-and-rolls while maintaining a good locker room presence. That will be appealing to any team with max cap space. For Philadelphia to keep him, it is almost certainly going to have to offer a five-year deal either at the max ($188 million) or very close to it.

Butler's market is a little more complex. He has a lot of miles on his body, and certainly had tumultuous stays in both Chicago and Minnesota. Yet he still remains one of the league's elite two-way wing players and, more important, he's a big-game player -- as he showed with his Game 2 performance in the second round of the playoffs against the Raptors to lead Philadelphia to a series-shifting victory. With several big-market teams holding max cap room and a desire to chase star free agents, it is hard to see Butler failing to get a max deal, too.

If Philadelphia was to re-sign both players and keep Redick, the luxury tax might be impossible to avoid -- and that's before filling out the bench, or giving Ben Simmons the max contract extension Brand has already said will be coming his way.

So, to keep everyone, Philadelphia will be signing up for the tax long term. But the alternative is losing pieces that were acquired at an extremely heavy price.

League-wide implications: In a summer when there will be so many big stars available, the future plans of Harris and Butler -- and, to a lesser extent, Redick -- will be of great interest to many teams. If Philadelphia makes an NBA Finals run, it would seem far more likely that the Sixers would be willing to spend the kind of money to keep this group together than if they lose to the Raptors.

Whether Harris and/or Butler stay in Philadelphia will not only determine if the Sixers remain an elite East contender, but it also will determine if the many teams around the league with max cap space have two fewer options to pursue.

Boston Celtics | 31 percent chance to advance

Biggest things at stake if they lose

1. Kyrie Irving's long-term home

Anyone who has even passively followed the NBA this season has been well aware of Irving's ups and downs the past several months. At every point Boston has seemed to get on track, there has been a moment -- often involving Irving -- when things have gone sideways again.

At the center of all that talk has been the subject of where Irving will be playing next season. Right now, things around the Celtics are as good as they've been all season. The team swept its first-round series against the Indiana Pacers, then opened its clash with the Bucks with an emphatic road victory Sunday afternoon.

Good luck figuring out where Irving will be next season, though. On any given day, the belief around the league could shift from him remaining in Boston to teaming up with Kevin Durant to play for his hometown New York Knicks or rejoining LeBron James -- this time on the other side of the country.

Whatever his decision winds up being will have wide-ranging ramifications for both the Celtics and the league -- and could easily be swung by how far Boston advances.

2. Less likely to trade for AD?

What happens with Irving could easily have an impact on the biggest domino waiting to fall: What happens with Anthony Davis this summer?

If Irving recommits to Boston on a long-term deal, the chances of the Celtics trading for Davis increase exponentially. In a vacuum, the Celtics easily have the best package to offer the New Orleans Pelicans in such a deal, between their young players (namely Jayson Tatum and Jaylen Brown) and a trove of draft picks this year and beyond.

But if Irving leaves, it would make the calculus for dealing for Davis far more fraught for Celtics president of basketball operations Danny Ainge and his front office. It is one thing to give up a significant number of young players and picks to pair Davis with Irving (and presumably Al Horford, who can also become a free agent this summer), forming what would likely be the best team in the East for years to come. It would be quite another to trade all of that for Davis with Irving out the door, significantly increasing the chances Davis is just a one-season rental.

Leaguewide implications: The fates of Irving and Davis are the two biggest questions hanging over the league right now (particularly given the widespread assumption that Durant is going to leave the Golden State Warriors this summer). If Irving remains and Davis arrives, the Celtics will become an immediate title favorite. If neither is in green and white next season, though, perhaps this season's Celtics will wind up being the most talented group Boston is able to put together anytime soon.

And two more of the top 10-15 players in the league changing teams would be a monumental shift of power, wherever they'd end up.

Milwaukee Bucks | 69 percent chance to advance

Biggest things at stake if they lose

1. Giannis' supermax countdown clock

The Bucks had a brilliant regular season. They won a league-leading 60 games. They won a playoff series for the first time in 18 years. Giannis Antetokounmpo will likely be named the league's MVP, and deservedly so.

But for all that success, Milwaukee has a weight hanging over everything for the next 14 months: Is Antetokounmpo, who might soon be the best player in the world (if he isn't already), willing to sign a supermax contract extension to remain in the Upper Midwest for years to come?

If he is, then the Bucks can breathe a sigh of relief and continue to build around one of the league's elite players. If he isn't? Well, then it is time to start to consider trading him -- in what would, arguably, be the most consequential deal in league history since ... the Bucks traded Kareem Abdul-Jabbar in 1975.

Every game Milwaukee wins this season -- up to and including an NBA Finals appearance and a title -- would help in the Bucks' quest to prove they have the talent necessary to win long term. But a second-round exit would reinforce many of the doubts that have existed all season long about the viability of this group to be a true championship contender. And that could make it more challenging for Milwaukee to make a case that this is Antetokounmpo's best place to win for years to come.

2. Is this core worth the price?

The other thing hanging over the Bucks right now: the upcoming free agencies of several of the team's key pieces. How many will stay, and what will it cost to keep them?

While the Bucks mitigated one potential free-agency issue by committing to a contract extension with Eric Bledsoe during the season, several other core players -- including All-Star Khris Middleton, starters Malcolm Brogdon and Brook Lopez and reserve Nikola Mirotic -- will hit the open market this summer.

In theory, the Bucks can keep all of them -- but doing so would almost certainly send Milwaukee soaring into the luxury tax. That is something ownership is willing to do, sources say, depending on how far the team goes this season.

A trip to the NBA Finals would make going into the tax an easy call. Going to the conference finals? That's a tougher one. Losing to the Celtics? Tougher still.

And given that all of this goes back to the goal of trying to recruit Antetokounmpo long term, it makes the decisions Milwaukee makes this summer all the more vital to get right.

Leaguewide implications: If Antetokounmpo were to turn down an extension, it would be a seismic event like few the league has ever seen.

In the meantime, though, the biggest Milwaukee domino for the rest of the league to watch is Middleton. A proven playoff performer and a consistent scoring threat in a league in which quality wing play is a premium commodity, Middleton should be able to command a max salary this summer -- be it from the Bucks or elsewhere.

Toronto Raptors | 69 percent chance to advance

Biggest things at stake if they lose

1. Toronto's case to keep Kawhi

Last summer, Toronto chose to trade one extra season of DeMar DeRozan for the chance at a championship ceiling with Kawhi Leonard. Toronto's decision has been validated through the first seven games of these playoffs. Leonard has been sensational, looking every bit the player he was before he spent all but nine games of last season sidelined with tendinopathy in his left quadriceps.

The question now is if Toronto's decision to trade for Leonard will work out like the Oklahoma City Thunder's did in dealing for Paul George a year earlier. Leonard has one of the tightest circles in the league, and his true thinking is something few people actually know intimately. Still, it is assumed his choice this summer will come down to one of two teams: the Raptors or his hometown LA Clippers.

George chose to stay in Oklahoma City despite losing in the first round last year, so it isn't exactly a guarantee that if Toronto loses now or reaches the Finals that it will sway Leonard's decision. Still, the Raptors will undoubtedly feel stronger about their case with each game they manage to win over the next several weeks.

2. Full-scale rebuild?

If Leonard chooses to stay, the Raptors will proceed forward with him and Pascal Siakam as the foundation of their team for the next several seasons -- a combination that will easily be good enough to keep Toronto in championship contention for the foreseeable future. But if Leonard leaves? Well, the Raptors will have big decisions to make.

Among Kyle Lowry, Marc Gasol (assuming he picks up his $25 million player option), Serge Ibaka and Fred VanVleet, the Raptors will have $91 million in expiring contracts in 2019-20. They could simply try to be as competitive as possible next season and then let their books basically be wiped clean to chase free agents. Or Toronto could use those contracts as trade bait to bring in new talent to surround Siakam with moving forward.

Then there's also Danny Green, who shot 45.5 percent from 3-point range this season and remains an elite defensive option on the wing. Given the paucity of 3-and-D players in the NBA these days, Green will be highly coveted this summer. If Leonard stays, it would make sense for the Raptors to pay what it takes to keep Green. If not? He'll be in high demand.

The trade for Leonard last summer was just the latest example of Raptors president of basketball operations Masai Ujiri being willing to swing for the fences and get creative. Whether Leonard stays or goes, Ujiri isn't going to stop taking risks moving forward -- maybe as early as this summer.

Leaguewide implications: Leonard's free agency has been a constant topic of discussion for more than a year. It will remain one until he makes his decision, and whether one of the league's elite players chooses to stay in the East or goes out West could have a big impact on the league's power balance -- which, thanks to the rise of these four teams, finally appears to be leveling out after decades of West imbalance.

While Lowry and Gasol are entering their mid-30s, they remain very good players who would draw interest on the open market if Toronto chooses to move on. And if Green leaves Toronto, he would fit with just about any contending team given his ability to both shoot and defend on the wing. Given that all of them will likely be in Toronto if Leonard stays -- and perhaps none of them will be if he goes -- that decision could have a huge impact on the 2020 NBA championship.