Over the last 35 years of wrenching, I'm sure I've cross threaded something on every day of the week at some point.

 

It's just not nice to use those words on Sunday.

 

Maybe the voltage firing the #4 plug is a bit weak? Back when I had my race car I had a Snap-on voltage reader that could read the voltage at the plug which totally eliminated any guess work on what was happening. That was their unit for the old fashioned coil/distributor/plug wire type of system but now they also have one for the newer types of systems we have now. If possible see if you can find someone that can read the voltage for you?

On my Porsche 2L race engine, plugs would get fouled if voltage dropped to 12,000 or so from the normal 16,000 volts but you could never hear or feel it so it was a race day type of check.

 

The original coil on my '85 Dodge got weak. I didn't even notice it much. The thing was underpowered to start with. To pass anybody I had to wait until I was at the top of a hill and had a tailwind.

Anyway, it finally got to the point where it would barely run when it was raining. A new coil and it was back up to "full" power again.

A newer Dodge kept giving a MAF code. A new part didn't help but a pigtail out of a junkyard did the trick. The sockets in the plug had stretched and kept giving an intermittent code.