OregonPlinker has a very good point. It is a very common modification to put a 4.0 head on a 4.2 block. But normally the reason for doing so is to get the fuel injection. Seems odd that the 4.0 head was used and they put the stock manifold back onto it. So that you still have a carburetor.
Does it have an aluminum valve cover?
Most of the 4.0's have a plastic valve cover, not all though. Some came with a stamped steel cover. If its aluminum its either a 4.2L or an aftermarket valve cover. Atleast thats what i have found from my many dealings with the 2 motors.
But, if the motor is original, for an 89, its a carbureted 4.2L.