number 4 cylinder miss fire! on 2008 Mazda MAZDASPEED3

I keep getting a number 4 cylinder miss fire. I have changed the plugs and switched the coils. I still get the same problem, also I've noticed that there is oil on the number 4 cylinder spark plug treads, is this bad? I just had this motor rebuilt last year with forged internals, please help.

Asked by for the 2008 Mazda MAZDASPEED3
sounds like a valve cover gasket is needed along with a boot , and the treads mean its misfiring.