First, what are the symptoms? If there's only 1 injector problem, then I am guessing that the engine is running rough?
Is it constantly rough or better at higher rpms?
+/- .5 mg/str is an acceptable difference.
You could have a bad injector. You could also try removing the bad injector and putting it in another slot. If the bad injector is still #4, then it could be something else, if it stays with the moved injector, then it could be a bad injector.
You could also try a can of diesel purge cleaner to see if that evens out the problem.