[HO455]

Welcome aboard! Sorry to hear about your difficulties. It sounds as if you've checked the ignition and fuel delivery pretty thoroughly. Like Dashman says you may have a head gasket problem although #1 & #4 aren't on the same bank. Generally you see problems in adjacent cylinders. Doing a leak down test with the engine hot or warm should answer that question.

You didn't mention whether there is any valve train noise.. You may have some bad or really worn lifters that are bleeding too much oil when the engine is hot. The valve adjustment you found cranked down may have been someone trying to fix the problem. I suggest checking the amount of rocker movement on #1 & #4 cylinders compared with one of the cylinders that are at temperature. You may find a valve with low lift.

Another quick test would be to try adding some STP or putting some heaver weight oil in and see if the problem changes.

Good luck and keep us updated on your progress. And and you should know from lurking here we love pictures!