Well if he cant follow house rules then I would say they need to stay downstairs when they are over at your house. Or I would tell him that the girlfriend can not come over unless they can follow rules. I am 20 and I have an 18 month old son. I did the same stuff he is doing. It only takes one...
There is no way to know four days after sex. You will have to wait until your next period is due and take a test if you miss it.
I wouldn't worry so much until you know for sure and just be more careful if you are going to continue having sex.