I have a 2004 f-150 5.4 liter. I am getting a missfire code on number 4 and a Coil D primary/secondary circuit malfunction on 4. I switched coils and am getting the same codes so its not the coil. Can it be the plug? any help would be great I am stumped.

 

Try switching the plug depending on miles since last changing the plugs.

Also may depend on how many miles you drove once you switched the coil. (if you swaped out coils, cleard the CEL, then drove the truck enough to have the CEL come back on & scanned again to be sure you are receiving the same code).