I doubt there would be a criminal penalty for something like that.
While it's immoral, my family has been known to do it once or twice.
![Eek! :eek: :eek:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
I stayed a kid until I was like 19, but now I'm an adult. We've never had anything said to us. If anything were, then you could always straighten it out there by telling them that the extras just decided to come along after the initial reservations were made. The worst thing that will happen is that they'll charge you the extra $10. I see it as no different than those who pool hop when they're not technically supposed to, or those who bring back the mugs (which clearly state that they are valid only on the original stay) and refill them on all vacations.
Is it immoral and wrong? Yes.
Do many people do it anyway? Yes.
Oh, and also, if you don't tell them four adults, you won't all get room keys. Obviously you only get room keys for the people they know are there.