I don't think there are very many parts that will interchange to be honest - I believe the chassis on the xxx-4 is longer than the xxx-s, so the belt wont work, maybe other drivetrain parts like the diffs/outdrives, some misc hardware like screws and washers, maybe a hingepin along with an arm mount or something.
You would think that 2 cars that are virtually identical would share a lot of parts, but I guess these cars are an example of those that don't.