My girlfriend and I have been dating for four years. Today out of nowhere she said we have to get married. I said okay but give me some time. she said it is now or never. I said not now and she said we are over. What should I do?
You should believe her. There was something leading up to it I'm sure, she's tired of waiting thinking you might have promised more than delivered? Only you know, but I'll bet she has told you this before and you chose not to listen until she said do or die, because she's not going to get old with someone who has no clear intentions. You aren't ready, she is, you can't make her unready, so get ready or get gone!
4 years is a lot of time invested. If you are over 25 and you feel she is the one you want to spend your life with, I suggest you go out and buy a ring.
If you are under 25, I suggest you talk about it some more. You both still have some growing to do.
If you can picture your life without her, maybe it is time to move on. Don't stay with someone because it is comfortable, stay because you love them.