1998 Subaru Legacy Check Engine P0304

I'm getting a check engine code P0304 for a mis fire in cylinder 4. I have replaced the spark plug, wire, and tested the coil with meter and all is fine.
I could tell from engine vibration if it was mis fireing. At first it was very sporadic. It would be fine for a day and then it would mis fire again the next day, and would work fine for half of the next day. Now it has gotten worse and I believe cylinder 4 is not working at all because engine vibration at idle is really bad(whole car shaking) and the car is on the edge of stalling at idle. I just don't know what to do next but to take it to a professional repair place and spend buku bucks just for them to spend hours trying to find the problem and replacing all my electrical, again. Please help!