I've got to say I agree with most of the other posters here. Without reiterating what they've said, think about this one.

What could go wrong if you didn't ask her; vs what could go wrong if you did ask her?

If you get engaged now it's not that big of a deal, but when you get married is when it becomes a bigger fish to fry. If you get married while you both are changing you could end up losing the bond you once had and things could come to light that you never thought of. Then you're looking at divorce, possibly custody issues...it can just get really messy. However if you get engaged but don't actually marry until your mid twenties, things might be a little clearer for you. Have you two already been living together? I'll tell you, I'm almost the exact opposite person I was 4 years ago. It's just plain scary, and even at 22 I know I have a lot more living to do.

Hopefully whatever you decide to do works out the best for both of you. :]