DoDo1975

After replacing my injector on cylinder 4, car ran really smooth for a few days, then started to get worse and after a few days was running rough again. Code also came back (Misfire on 4). My car runs rough at idle and lower RPMs, not to the point of stalling ,just rough. I also get 30% worse fuel economy than I should. I want to determine what to check next. I have read MAF sensor and also the coil pack. Does the fact the car ran fine for several days while the computer adjusted eliminate the coil packs, or point to something else? I don't want the stealership blindly replacing things and charging each time. Its a 2005 Subaru Legacy GT with 136,000 km.