So right now the standings are
4) NYL 37-30 (@LAC, @LAL,vsSAS,vsCHA, vsBOS)
5) ATL 37-31 (vsWAS, vsWAS, vsORL, vsHOU)
6) MIA 36-31 (@BOS, @BOS, vsPHI, @MIL, @DET)
7) BOS 35-32 (vsMIA, vsMIA, @CLE, @MIN, @NYK).
So what needs to happen?
BOS plays NYK the last game of the year, if they win that they also get the tie breaker. So they can still pass NYK if they win one more game than them in the next 4 games AND beat them the last game of the season. NYK has some pretty tough games left against both the LA teams and CHA/SAS who are both in the playoff hunt.
To pass ATL, BOS needs to lose 2 fewer games than ATL the rest of the year because ATL already has the tie breaker. This is a tough one because ATL only has four games left. Two of those are against WAS who is playing VERY well and still chasing positioning in the play in round.
BOS plays its next two against MIA. If BOS sweeps they will move one game up on MIA WITH the tie breaker with only three to play which would nearly lock in a better finish than MIA. If BOS splits the next two with MIA then BOS get's the tie breaker, but BOS would need to lose one fewer game over each teams last three to pull even and pass MIA with that tie breaker. Not impossible given MIA plays both PHI who may still be trying to lock up the 1 seed, and MIL who will be trying to lock up the 2 seed, in their final three games.
TLDR: So basically its gonna be hard to pass ATL unless they WIN out. They can lose MAYBE one more game (say a split against MIA) and still stand a chance of passing either/or NYK or MIA as long as they win all their other remaining games (including the last game against NYK) in part because NYK and MIA have some tough games left and BOS would own the tiebreakers.
BOS still kind of controls its own destiny here at least in terms of getting to the fifth seed (need one more NYK loss).