I was just sitting at a light and then heard what sounded like a misfire. It would stumbled off the line then keep "shaking" the car then check engine light went on. I thought it was bad gas so I filled up with 93 and reset my ecu by unplugging the battery and it still does it. I went to o'reilly's and used the coed reader and it said cylinder 4 misfire. What could it be? (replaced the sparkplugs with my friend 6k miles ago so maybe thats it?). Its an 04 5at with 55k.
Thanks!

if its just that cylinder its probably a coil. pull the plug out and see if it looks wet and dark thatll mean it isn;t firing or if its dry and white thatll mean no fuel so bad injector