i have a 87 rx7 with single piston brake calipers. Will the 4 piston caliper from the same car fit with no problem? or do i need something else to make it fit?

 

check bolt distance and make sure it actually covers the disk with pads,
you dont want any steel rubbin in there (500F)

 

if you already have 5-lug hubs...yes. If not it would be easiest to just get the 5-lug hubs and brakes and install them at the same time.

 

check your rear brake rotors and see if they are solid or vented
also since this car is NA there is really no reason to upgrade to a set of 4 piston calipers unless your tracking the car on a road course
I would also check and make sure the master cylinder is the same for both