Couple things.

Even though you "hear " the sound from the top end, check the engine driven vacuum pump, confirm that it is one of the upgraded type, and with you're stethoscope give it a good listen. Any failure of the vacuum pump will likely create a noise that will vary with engine RPM.

You thought you detected the "tick" at the #4 injector, then you fooled with the injectors, is it now that the "tick" you heard earlier is no longer detectable at the #4 injector? Were the injectors returned to there respective cylinders? Any chance the "tick" has moved to another cylinder?

When you identified a "tick" at the #4 injector a good next step would have been to move that injector to a different cylinder and then observe whether the "tick" followed the injector or remained with the cylinder.

Injectors can act differently under power and at speed were a bench test simply verifies the pop pressure and spray pattern mostly.

If the lifter is the source of the noise it will be more difficult to identify, you'd mentioned after the oil change the sound was altered, there are numerous examples of lifter noise in particular being effected by oil choices, usually using a good quality synthetic oil such as MOBIL 1 will lessen or quiet lifters that are noisy but not completely worn out.

Assuming the "tick" is not unbearable or getting worse a prudent course of action would be to try some high grade oil and see if that makes any difference, it won't happen instantly but you'd know after a couple hundred miles if it has any effect. Personally I would avoid any of the Snake Oil additive stuff until you figure out if just running the oil spec'd by MB will make any difference. You can drain what you just changed and hang onto to it, it might look black but it's not really "used" and useless.