OK so our 2002 GMC envoy just started misfiring really bad. Ses light is on for code p0304 misfire cylinder 4. Changed all the plugs and found a pool of oil on top of plugs 3 and a lot on top of plug 4 (from front) . I cleaned all of the oil up best I could and also changed the coil gaskets. Changed the plugs and ses came back on p0304. I'm thinking it has to be coil or injector right? Can I swap plugins on coils to see if it changes cylinder?