B5m diff and b4 diff
 

Is the b5m diff the same size as the b4 diff.

No it doesn't.

The B4 outdrive is shorter than B5M

Just the out drives then?

would expect that the bearings you need will be different as well.
the B5 outdrives sit in metric bearings.
the B4 would presumably be imperial.

It's never simple :o

The original B4 is imperial
The B4.1 / B4.2 are metric

And the B4.1 / B4.2 metric out drive bearing is a different size to the B5M outdrive bearing so not interchangeable.