The engine is not running on 4 cylinders

I would start with basics. A compression test. Fuel pressure check, spark test.

The injectors need to be verified that they are firing and allowing fuel into the cylinder. The distributor needs to be checked as well that it is passing the spark to the cylinders.
Check for vacuum leaks as well. The intake is a common failure for its gaskets.
That would be my starting point.

Roy

Ok, you still need to look at injectors and a vacuum leak. Did you put a vacuum gauge on it?

Roy

Let us know how you make out

2, 3, 5, and 8 are supplied power by injector fuse 5 in the underhood relay center. Check this fuse and more than likely you will find it open. The ground side of each injector is controlled by the PCM through individual drivers. A faulty ecm would most likely result in one, or another cylinder with no injector fire. If you find the fuse open, either the power side of that respective bank of injectors is shorted, or most likely an injector(s) is faulty pulling too much current. Check the resistance of the injectors(all 8) and you would look for 15 to 17 ohms being good. Any less need replaced. 1991 did have injector problems.