Well if you haven't seen her in four months I think you should be careful about getting your hopes up; especially if don't know her well and you've been thinking about her alot... how she is in your head might not be how she is in real life.

Anyway, I think it's better not to seem too interested in her relationship-wise if she has a boyfriend. Maybe you could try and arrange a meet-up with your mutual friend and ask her if she could invite this girl too, and maybe take it from there.