Division winners get the top 3 seeds.
Here is how they do the seeding now. Celts can still get 4th seed if they win the Atlantic:
To address the criticisms of having each division champion guaranteed a top-3 seed, regardless of record, the rules were changed such that the division winners are now only guaranteed a top-4 seed. The best second-placed team in the division now may be seeded as high as 2 depending on its regular season record. This avoids the scenario of possibly having the two best teams in the conference seeded 1 and 4 if they play in the same division, thus being forced to play each other in the second round (given no upsets).