if its only cylinder 4 and the plugs are defiantly clean and NOT wet I would get a compression test done to check for burnt exhaust valve, blown gasket and rings. could be something as simple as a plug lead thought, i would get the compression test done first to rule out the expensive fixes, also check the injector on cylinder 4 hopefully it gets fixed easily for you. my car did this and turned out to be a faulty lead… and the timing was way out also so it misfired and banged like a gun.