Something to consider-

The last two times I've flown UA and checked a bag (currently VA silver, no baggage allowance), after checking a bag the kiosk went to a new screen asking if I wanted to check my 'carry-on' for free. Presumably it thinks you are done checking your big bag and the airline wants to encourage you to not stuff the overhead with a second bag, if you have one.

So, next time my partner and I were flying and intending to check a bag each, I checked mine and then at the second prompt, checked her bag for free. There is no difference between the bag tag either time. So for the OP that checked 4 bags, this could save some $$....