Prior to my annual I was experiencing backfiring whenever at low rpms. Either on startup or whenever pulling the power to enter the pattern. On start up it would clear pretty quickly. In the air everything was fine. Enter the pattern and pull power and you got the unwanted few pops. On day my A&P pulled the cowl and found number 4 to be cold shortly after startup. We changed plugs and all was well. It started up again so at my yearly check up we chose to replace the rear mag plate with new plug wires included. All plugs were cleaned and recapped. The next 4 flights were uneventful. After my last 20 min flight the plane sat for a few hours and on restart the is toms returned. After several attempts over several days number 4 is as cold as ice. The engine runs very rough and will not run over 1700-1800 rpms. On hand turning the prop compressions seem to be normal on all cylinders. Any thoughts? Stuck valve(s) perhaps?