Yes, you can use the 91 4cyl calipers on a solid axle. You must use them with a vented LandCruiser rotor from a 79-83 FJ40 for them to work.
In a perfect world, you'd use the V6 calipers, as stated by dieselsmoke. I'm using the 4cyl calipers on my rig and they seem to work fine...
Ryan
