code 0304 came on today?

hi corsa 1.2 53 reg started the car today and engine light came on saying 0304 cylinder 4 misfire but after turned engine of and tried again it went away so i took plug out its black its always black had this problem for ages now any way looked down plug hole its all wet so i scooped a bit up with a screw driver and it smells of fuel im thinking now could it be the fuel injector any one thanks