Have you checked the plug, lead, cap and rotor arm are alright? If it's not got issues on the other 3 cylinders then electrically all it could be is one of the above 4 parts.
Swap a plug from another cylinder and see if the misfire is still there, has moved, or is now in both.
Assuming nothings changed, swap the lead with another cylinder (remember to swap them round on the dizzy cap too). Test again.
Assuming the misfire still hasn't changed, I'd be looking at the terminal in the dizzy cap, if it's badly worn or chipped, etc it won't make a good contact as the rotor arm passes = weak spark.
What were the results for each cylinders compression?
Could it be a bent valve? Have you had issues with timing (especially since any work has been done that could point to something?
I'm assuming you're running standard ignition and not an aftermarket coil pack or something....