can you tell which one is missing..
im assuming that one cylinder is not firing right.
put it under load and see which plug it is.. either with a plug checker or by pulling wires..
to put it under load put it in 2nd and brake torque it..
i taught the local mechanic that after i told him i had a missfir but only when i pulled a heavy trailer( trailer w/ bobcat)( oh this was in my superduty) he wanted me to bring my trailer and bobcat so he could diagnose..
that was funny i laughed at him for that one..
see if its just one plug if it is maybe you had a ring go bad check compressions on all the cylinders...
you could have a cracked plug..
when my superduty was missing thats what it turned out to be. he changed the plug and it was fine....
just random thoughts..
are all the plugs colored the same?
i guess it needs to be narrowed to either an all cylinder problem like fuel pressure being off or a 1 cylinder problem. plug ring valve rocker broke?
narrow it down...
put it in the dark garage and load the motor see if you see spark jumping anywhere..