"Once a cheat always a cheat"
Probably hard to deal with all this, I know man, I had a girl cheat on me a few years ago. Usually a girl will leave out a few details. When my girl cheated on me she told me she only kissed the guy, but her best friend said they were having sex in his bedroom. Bad memories. For some reason (desperation) I took her back and they were the worst 11 months of my life.
Cut your losses and just dump the bytch. You have a gold mine of a site here, and there are plenty of women in the world who would be better for you. You should have NO PROBLEM replacing her.
Trust me, if you stay together, this will be in the back of your head ALL THE TIME. It will eat away at you like you would never believe. Came home at 4:30????? That's f'ed up. She obviously has NO RESPECT for you. It doesn't matter how bad she feels, the fact is that she did it. If she respected you she wouldn't have done anything with this guy.
It's probably true that it's easier to get a new girl than it is to keep one. You probably just found this out though. Look on the bright side though, you know that girls aren't exactly always faithful to their boyfriends. This can come in handy for when you are single!