Do a compression check on the engine. The #4 cylinder may have a leaking intake valve. The engine runs smoother when you pull the sparkplug wire off because there is no longer exhaust getting forced out of the #4 cylinder and into the intake manifold, where it screws up the rest of the cylinders fuel/air mixture. It would be like opening up the EGR(exhaust gas recirculation) valve at idle, the engine runs like crap because of the diluted air/fuel mixture. Good luck with the V6.