Hi glassetcher and welcome to the forum.

The 4BP will provide a more diffuse sound field than the 4B would.  This will create a more immersive soundstage for movies when used as surrounds.  The 4B's work well for surrounds too, but the 4BP will give you a more immersive effect due to it's three sides.  You could also go with teh 4BPs for rears too if you desire.  However, if you also intend to listen to a lot of music, I would perhaps leave the rears as 4Bs since the directional 4Bs are more desireable for music.