On one of my truck engines which was used only in the winter season, but sat for the rest of the year, I got some real bad gas that made the valves sticky. The valves would follow the slow RPM of cranking, but a few of them would hang up enough to give similar symptoms to what you have. They could not follow the cam quickly enough, but esentially "floated" at any running RPM.
One possible test is to remove the plug from #4, and start the engine (running on 1,2,3,). Put a finger over the plug hole and see if you get sharp pulses pushing your finger away from the head. At idle, you will probably feel the intake stroke pulling your finger tighter into the plug hole. After the finger test, I get braver (maybe more stupid??) and get out the Burnzo propane torch. Light it and play the flame over that open spark plug hole with the engine running. You will get a sharp "bark" on every compression stroke if the intake valve is working correctly. And I do know that the gas tank is just above the head, so I use a sheet metal deflector plate as a heat shield when doing the bark test.
I don't thing the above tests will tell anything about a sticky exhaust valve, which would also render that cylinder useless. The quick fluctuation of a vacuum gauge needle would give an indication, but your intake manifold may not have a vacuum port (some did).
I'm beginning to accept the mud daubber theory, follow up the exhaust pipe and the first cylinder they'd come to is #4. I have fought with the mud dams these buggers make in engines and RV LP gas and sewer vent systems even up here in the Siberian northland. Maybe it is time to pull off the manifold and do some inspecting.
Paul in MN