I take it you are out of warranty?
My guess is that it's the wiring harness.
The clips tend to snap on the coil packs. We've seen misfire codes that won't go away after a 60k service due to the clip snapping. We finally wind up supergluing them and they are fine.
If you've moved around stuff like injectors, plugs, etc. Then it could be:
Wiring in the harness that go to cylnder #4, or a sticking valve perhaps, or ECU.