I meant to add to my last sentence, don't totally rule out the head gasket. You would normally expect that to affect compression, but not always. Was your uniform 150 psi done wet or dry? Check oil for any hint of milkiness (dipstick and filler cap inspection) and coolant reservoir for cloudiness, dark color change or oiliness. Any unexplained loss of oil or coolant more so than before? Old engines are going to leak, so the question is if it's notably worse since the engine started running poorly. You've got a compression gauge, so a leakdown test is warranted.

I also meant to add as far as plugs not sealing the possibility of being cross threaded. #4 plug is the hardest to see and easiest to accidentally cross thread if you don't hand thread it in far enough. A cross threaded plug may feel snug, yet not be fully seated, even with a torque wrench. Also, thread damage may not be evident on the plug, especially if it was a previous plug that cross threaded. Using a different plug, thread it in part way. Does it wiggle more than if you do the exact same in the other plug holes? Valve cover leaks are also more common at the corners.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now