I'm speaking B4 generically. B4-2, B4-4, etc. There was even a B4-0 at one point.

The B4-x probably started out as a relabeled B.8-x, and the A5-x as a relabeled A.8-x. Old motor technology items.

Eventually the B4-x was redesigned to use the newer motor technology, and the A5 dropped.