BM4X4 is an improved version of BM45.
IIRC, the BM4x4 is a fairly quick fermenter. I could keep it fairly cool down there which might slow the ferment. On the other hand, I do like the RC212 - used it in many of my reds, but don't remember it being as fast as the 4x4.
I've got a little time to think about it. I only have one 6 gallon glass carboy available. I'd like two - one for the PN and the other to transfer the wines I have aging - before kicking this off.