Okay, I talked with the mechanic today. Here is what he tried. 3 different crank sensors shimmed 3 different ways, multiple crank position relearns, 2 different cam position sensors, 2 distributor caps, changed plug on offending cylinder, changed wires on cylinder, changed poppit valve on injector spider. Nothing changed the symptoms. Still cyl 4 misfire above about 1500 rpm. I found another post on a forum today with exactly the same symptoms, but they never posted a solution. My friend who is a 30 year chevy mechanic at a chevy dealership doesn't have a clue either. So I am driving it with a flashing SES light.