If you have access to an air compressor you can put the #4 Piston on the compression stroke and pressurize the Cylinder with Filtered compressed air. Afterwhich, you will listen for an air leak; maybe with a Mechanics Stethoscope.
If Intake or Exhaust Valves are leaking you should be able to hear Air leaking at their Manifolds.
You could also try this incase the Rings are stuck on that Cylinder.
http://www.peachparts.com/shopforum/showthread.php?t=258757&highlight=Marvel