Q: Playing empty/last groove ?

I'll be interested to hear what others say, but I would imagine it wouldn't do anything "to" the cartridge that 4 hours of normal play would do. It may have caused extra wear on the empty groove given the heat that would build up, but does that really mater?

Objectively, four hours is four hours. In this subjective world, the ill you caused is only limited by your imagination.

Clean your stylus well to remove the layer of burnished-on vinyl particles and crud. It's there - guaranteed.

Use something stronger/more abrasive than a dry brush, goop or gel. Search for "magic eraser" if you need a cheap and effective idea or use a sliver of very fine sandpaper, like Linn has always recommended.

Don't worry. All you did was use 4 of the thousands of hours worth of objective playing time on your cartridge. Subjectively, you didn't even notice! ;-)