Hello, MT-2500 you do have the patience of JOB.
Question 1)
I would like to know if any of the test results were with a hot engine, you know, at proper operating temp.
My basic knowledge is that the computer will always feed additional fuel to a cold (colder) engine to compensate for poor fuel atomization in the COLD air stream.
If any sensor is telling the computer the engine is cold (colder) it will compensate with additional fuel.
This can cause an uneven idle and poor fuel mileage.
If you can't get the engine up to temp changing out thermostats, try covering the radiator with a blanket until you get at least 195. Then run your tests at temp.
Question 2)
Is a cylinder leak down test appropriate to see if we can determine compression leakage at an intake or exhaust valve (I know this is old school, but so am I)??
This test might reveal a reason for low speed misfire at #4, although I am inclined to believe it is an intake leak at #4.
A slight valve leak will be far more evident at idle and will tend to disappear to speed. It may not show up in a compression test (wet or dry) depending on the engine temp at the time of the compression test.
Engine should be at operating temp (195), pull ALL of the plugs, disable the fuel pump prior to the test. I had a friend who made this test with the pump connected and the ignition working and had to go to the hospital for facial burns.
Question 3)
Did you put in a Dephi pump or some other unit? What were the pressures as described by MT-2500 in his test sequence?
I have found very common problems with every brand of knock off pumps, from poor electrical connections, to low flow and pressure, to leaks in the body, all of which are hard to test for while in the tank. Use the more expensive unit, since you have thrown a ton of money at this vehicle in every other way.
Please post results. Thank You MT-2500. You are the MAN.