Intermittent po304 cylinder 4 misfire

[Antslater1]

Hi guys 

I am New to the forum I am  looking for some advice keep getting flashing eml I read ecu and get dtc po304 cylinder 4 misfire I changed the plugs and erase dtc car run ok .3 or 4 days later same thing happens so I swapped no 4 coil pack with no 3 (assuming no 4 is at gear box end of engine) again 3 or 4 days later get flashing eml I read ecu and get dtc po304 cylinder 4 misfire this now rules out a fault with the coil pack

when changing plugs i noticed a slight misting of oil from rocker box cover down the plug cavity would this cause a problem or should I be looking for a wiring fault ? 

Thanks in advance 

Antslater1