121 k miles ,, Code is still the same after installing Brass insert Cap and rotor, moving plug wires from cylinder to cylinder, moving spark plug to another cylinder. Is it possible that distributor drive gear is worn to cause the same misfire #4 cylinder? Vehicle does have the idle shakes, but runs fine off idle and at higher RPMs.

might have popett valve sticking on #4 injector,

Is there a test procedure for the popette valve? Is it possible to run injector cleaner in the tank to clean the popette valve?