2000 Volkswagen Jetta Code P0304 - Misfiring in the fourth

Friday morning I turned on the car and there was a slight jerk back and forth coming from the car. When I went to pull out, the Check Engine light was flashing and then went to solid. I drove the car to work with the light on. I later drove it back on the highway in second gear at about 10mph (because of a traffic jam). Once the traffic jam cleared up I accelerated and the car started to jerk a lot and the Check Engine light was flashing. I pulled over for a minute and then drove the car home no problem.

I tested the car and I got the code P0304 - Misfiring in the fourth cylinder. Would this be an o2 sensor problem? or ignition coil? or spark plug? Thanks.