For average number of pieces captured I get 3.8873.

This is a bit tricky because capturing 4 pieces, for example, could happen like M,M,M,M,B or M,M,M,F.

I would add this somewhat counter-intuitive tidbit. The probability of capturing a certain number of pieces, in order from most likely to least likely, is: 0, 1, 4, 2, 5, 3, 6, 7, 8, 9, 10, 11, 12, 13.