97 Z-71 troubleshoot misfire

97 Z-71 K1500 with 150K miles. Service engine soon light stays on and if I idle too long, it begins to flash. The code is for a #4 cylinder misfire. It used to be that if I ran it on the highway and didn't idle too long, the light went off but now it stays on continuously. Compression holds at 140 psi on #4 cylinder. Fuel pressure is 60 psi, relatively new wires, plugs, distributor, cap, rotor. All AC Delco. New fuel regulator and injector on #4 cylinder. Checked with a spark plug at the block and it's making fire. When I pulled the wire from #4, it really didn't sound worse than it always does. Any ideas why it still runs rough and is misfiring?

I would do a leak down test to eliminate a mechanical issue, remember though, a bad valve spring or worn cam lobe will not show up with this test. Check for vacuum leaks in that area of the intake manifold also.

How do I do a leakdown test on that cylinder?

With a special tool setup something like a compression tester, fairly expensive but a much more comprehensive test then a compression test, anything above 15% is usually a problem, you will here the air escaping into the intake, exhaust, or adjacent cylinder. It uses shop air to maintain and read cylinder pressure losses.