Engine misfire on one cyl

I have an OBD code for an engine misfire on cycl 4.... I"ve swapped in this order, distr. cap, rotor, and plug.... then the wirse and plug again, and it still misfires.
What could be wrong.


P.s.
I do have a faulty O2 sensor that needs replaced also but I didn't think this would cause a misfire.... it's in bank three I believe.

what year is the truck? The 92-95 4.3L engines have problems with their spider injectors.

2000

Sounds like the same problem mine is having. No clue why.

Possibly a vacuum leak in the #4 runner of the intake manifold. Cylinder #4 is the center cylinder on the passenger side of the engine. Try directing a fine spray of carb cleaner along the intake and notice if the RPM increases and/or idle quality improves. Another trick is to put one end of a 3/4" heater hose next to your ear and move the other end around searching for the leak. When you get near the leak, you'll hear the hissing sound loud and clear through the hose. Hope this helps.