I've seen it just be as simple as a bad fit of the spring terminal on a 4.6, good coil, good plugs, just a loose spring end.

 

Is there a way to ensure a good fit of the coil on the top of the plug?

 

Just test fit the spring's plug end on a spare or used plug before install and make sure spring isn't kinked, etc ... and closely inspect the boot for tears or cuts or cracks.

Inside the boot is a long coil spring, it's not unheard of to have a burned through part. Likewise, if the boot itself has a tear or cut it can allow a random misfire as the spark jumps from that coil to the wall of the spark plug well.

Also, look for signs of moisture when you pull the COP off, there should be no moisture in the spark plug well. Always a good idea to coat the COP's boot outer lips with silicone grease at install to keep it compliant and sealing better. Never spray pressure washer at COPs or boots.

The springs and boots are easily removed and installed on the coils, and both are available in kit for new at places like Rockauto, NAPA, Advance, O'Rielly's, etc for less than a new coil.

 

I've done all above minus test fitting them. All 7 plugs are from autozone but number 4 is from napa. I doubt that that makes a difference but it could. Also when I pulled them all out they smelt like burnt gas and were the rubber met the plug were dark brown in color. Could it be that burnt residue causing my problems?