Since I will now be keeping my 4x100 setup on the ol GT, I'd like to somehow swap out the rear drums to disc brakes, but retain the 4x100 lugs. I'm wondering if I can replace the rear end bits, but then keep the 4x100 hub and then get another pair of front discs for the rear. I guess the big question is if the 4lug hub would fit the 5lug caliper mount bits?
To do a 4 lug disc swap, you'll need the rear brake set-up from a Corolla GTS or FX16. Good luck finding these parts. You can't use a front disc on the rear for many reasons; the inside of the rear disc contains the e-brake, the diameter of the front disc is smaller because it doesn't have the e-brake in the middle, etc... You didn't mention why you didn't want to go with a 5 lug set-up all around. 5-lug set-ups are a lot easier to find and probably cheaper than a set-up from a Corolla.
Which corolla? I might be able to source an AE86 rear. I'm surprised there weren't any others with a 4x100 rear disc on FWD. Do I need the whole rear setup, or just the disc? In theory I could just buy a new rear disc for an FX16 and use the rest of the parts from the 4->5 lug swap, unless, the hub is not compatible.. Basically I think it will be easier to find a rear wheel with the high offset I need in 4x100, whereas it has proven difficult to find in 5x100. I actually managed to find 3 wheels that will work for my setup, whereas for 5x100, nothing.
ae92 corolla years 1988-1991 gt-s or fx16/gt-s. or a 1990-1992 geo prism gsi would recommend basically the whole rear end. i don't know all of the components you would need, but you can find these in just about any junk yard across the u.s. have at least 2 complete ones in my local yard here. good luck!