service engine soon light on on 2005 Mitsubishi Lancer

Service engine soon light is on and I judst bought the car. I had the code read at jiffy lube and the code was po 304
the car was hesitating and running with less power. I t is a manual trans. What does this code mean?

Asked by for the 2005 Mitsubishi Lancer
number 4 cylinder is missfiring. it could be a plug, coil or injector. needs to be checked out for a proper diag and a bid for repair.