mis fire in #4

I have a misfire on the number four cylinder..I changed plugs, boots,coil packs and still sounds like the flubber mobile any clue what could be the problem?

1. Is this a 4 or V6? 2. Did you do a compression check? 3. Did you change the injector as it could be going bad and sticking. 4. Did you check the ignition module under the coils? Get back with some answers and someone should be able to give you some direction.