Not that I am experienced in the subject but I like to think that as you get older, age gap matters less. 4 years when you are under 20 is probably a lot, but maybe not so much when you are 30. You two are still in the middle. You are mid twenties and working at a guess; she is 19 and studying/working? Are you likely to share enough interests to make it last, or maybe if you are both after something more casual, you haven't got anything to lose?