This may be a dumb idea, but how about switching the connectors on the injectors in the throttle body ?? If the problem remains the same, on #4 and #8, it seems unlikely to be related to the injector output from the ECM. Just an idea
Gapping the plugs closer would maybe give a clue for spark power, but seems hard to figure with all the new parts you have in there.
The manual I have suggests that the EGR port in the manifold can be a problem, and may require cleaning, and that the EGR vacuum port on the TBI for the solenoid can also be a problem. I know you cleaned the EGR valve, but thought I would mention that anyway.
still thinking................