Converting from 6 to 4 isn't too hard. Me and my dad did it for my shortened commodore diff. You need a metal lathe to do it neatly.

I machined up plugs for the bigger holes in the axle face, and then using a BIG MIG, I welded them in (plenty of weld penetration). I then used the lathe to machine the faces smooth.

I made a jig that is used to space the holes after the first one is drilled. It's essential that the first hole is drilled exactly on the PCD for the other studs. I machined a line exactly on the PCD to direct the drill tip. I then put the first stud in, and used my jig to direct me where to drill the next hole. I keep going until all 4 are drilled. You need to use exactly the right size drill, otherwise the studs won't hold tight in their new holes. Near enough isn't good enough for this job.

Chris