I was on an AA 757 recently and 10 empty seats went out in F from STL to DFW. Some were probably empty because of misconnects and rescheduling, since it was during irregular ops due to the weather. But they didn't fill them just because someone who did not want to pay was charming.

Op-ups can happen occasionally for one or two people. The odds of a GA putting four more people in F without a kiosk, sticker, or mileage upgrade would be pretty slim, IME.