I have a 2018 f150 3.5l ecoboost. Code P0304. Rough idle. Low compression in cylinder 4. I changed plugs and ignition coils. What else could this be? I held a rag up to my exhaust pipe and it gets sucked into the exhaust. Would that indicate a bad exhaust valve. Or could this be a cam issue?

 

I'd do a leakdown and confirm.

 

Yea did that too.