Yes, the 3rds will swap back and forth.
The big differences between the two:
6 cyl side bearing bore is 83mm ID
4 cyl side bearing bore is 80mm ID
6 cyl differential bearing journal is 50mm OD
4 cyl diff bearing journal is 45mm OD
With a 50mm brg journal OD the 6 cyl differential is stronger and will allow a larger dia. axle shaft to be installed. Can get custom cut side gears for a 6 cyl ARB, don't know if anyone is doing larger spline sides for other lockers.
You can put a 6 cyl differential in a 4 cyl case with a conversion bearing.
The larger 83mm bearing will give a better side load than the smaller 80mm bearing, however the 6cyl to 4cyl side bearing will have a better radial load, so it would be stronger for the loads we put on differentials. But the problem with that swap is:
The 6 cyl 3rd has a bit more webbing on the outside, this will help keep the pinion from flexing as much as it does in the 4 cyl 3rd.
