1998 nissan maxima po 1320 and po 304 trouble code. i check the spark plug and ingnition call on the number 4 cylinder, all are good i swap them to number 2 and cleared the code it still say's number 4 misfire. notice a little hard to start and white smoke from exhaust could it be the injector? unplug them one at a time due not here engine change on 2 of them. help somebody please