At this stage you might just swap in another injector first. When you have the injector out have a look in the prechamber for presence of the ball or too much goop or carbon. If that does not do it you might have visually not picked up a sloppy clearance on an individual valve without measuing it with a feeler gauge. Also at that stage compare the cam lobe profiles on the affected cylinder for wear. I personally would just read the milli volt output from the glow plug harness first. If #4 was higher voltage than the others it would be the injector or too much fuel on that cylinder. If lower voltage the injector or problems with the prechamber. If the same voltage as the other cylinders a sloppy valve, worn cam,bad valve spring or something mechanical. Divide and conquer is usually the best stratagy with most problems rather than guessing. Just my opinion.