single cly misfire

First the cpi went out replaced it then 50 miles later the coil went out did a tune up ( cap rotor plugs wires coil ) all was great for 200 miles then ses flashes single cly mise fire cly 4 not the plug not the wire not the cap good spark swaped plug for one on the other bank to eliminate the plug misfire stayed on cly 4 swaped wire for one on the other bank misfire stayed on cly 4 warrentied the cap (to eliminate) misfire stayed on cyl 4 pulled the plenum off again gasket was oil soaked around cyl 1,2,4 ports at first I thought possible air leaks because of the gasket causing a lean mis but its a common air plenum so I dont know that it would matter a whole lot any ideas would very helpful