B4 wheels have a different offset than B4 wheels. The fronts are probably close enough that it doesn't matter much, but B4 rears will make a B3 much narrower. The thing to do is get the Evader Nitro BX rear wheels. They have the same offset and same axle size as the B3. I have lots (Tower had them on sale once in GREEN
for $1.55 a pair and I got a bunch to use for bashing.) They work fine. Here are the white ones:
I think they are actually better (stronger) than the Associated or Pro-line B3 rear wheels.