SOS 4000 Rear Axle Swap & Spare Parts

Just picked up a 4000 parts tractor with an SOS trans to use for parts on my 4500 and I'm curious what of the rear axle is the same. I know the ratio is different, but it looks like that is in the ring & pinion and the planetaries are the same - is this correct?

Assuming this is the case, can I swap the entire axle planetary/brake/outer housing assembly? The brakes on the 4500 are pretty well shot and it seems like that might be easier than pulling the 4500 ones off and then stripping them far enough to change brake pads. What I'm unclear on though is if the ring gear comes out with the outer housings, and if it does will I have to adjust backlash and whatnot? Will this be more or less of a pain than just replacing brake discs? I think I've got a leaky axle seal but otherwise the 4500 axles seem ok. Wouldn't surprise me if 4000 bearings and such are in better shape than the 4500 too - 4500 looks to be well beat on in it's life.

Different tangent - is there any difference between the 3000 & 4000 SOS trans other than the d planetary on the 3000? I've got a 3500 with SOS and I'd like to know if the 4000 trans has enough in common to be worth storing for spare parts down the road. The 4000 has a stripped input shaft and apparently a pump front seal leak but was otherwise apparently functional.

Thanks.

The 4000 axle assemblies will bolt the the 4500 center, the left side axle holds the ring gear in place.

Although there is a procedure for setting the bearing preload it takes a special tool and the adjustment is done in the axle housings.
I swapped the axles assemblies from my 4000 to a center section and gears from a 4200 back in 2000, still going strong, no excess back lash, no noise.
The one difference is 4000's have 3 brake disc on each side while 4500's have 4 brake disc, this would give a little more stopping power and longer brake lift to the 4500. The 4th disc was most likely added because of the extra weight of the loader and hoe.
If the 4000 has good brakes and you don't operate the 4500 on a daily commercial basis I wouldn't worry about not having the 4th disc.
My 4500 brakes where shot and I had a set of axles from a 3930 with good brakes, so I swapped them out.

Not sure if there's a difference between the SOS transmissions or not.

Thanks.