I've got the same problem (i.e. P0304 code) on my 05 GT. After doing some searching and doing some research on this site, I've noticed an alarming trend of #4 cylinder coil pack, injector, and OCV failures. Here are my problems. My car idles poorly, not shaking, but poorly. It starts off with a bad stumble, but when it reaches a certain rpm, it starts to run better, not perfect, but better. I've replaced the drivers side OCV, this "seemed" to have made the idle better than it previously was (maybe it was wishful thinking on my part), still threw a P0304 code. I've checked for leaks and can find none. Here is my problem that nobody else has seemed to touch on, it has a lazy start. What I mean by that is, when I start the car, it is not the crisp, on one crank, start, it seems to roll before starting (hope that makes sense). The first shop I took it too said the compression was low in that cylinder, but that was before I replaced the OCV, so, I may have multiple issues going on here. I'm going to replace the #4 coil pack next, and then the injector, but, after that, I'm out of ideas. Any other advice out there from the esteemed panel of experts (no kidding, you guys are a lot of help). Thanks.