Im sure this question has been asked but my GF of 4 years is turning 21!?

I don't think it's so much the money as the effort for things like this. Think about something like making a photo album or scrap book of photos, tickets, little snippets like that from the time you've been together, maybe with a little comment under a few of them, and decorate the pages nicely. She will LOVE it - on special occasions it's the special and romantic things which will win her over, something that shows you really know her and that not just anyone would think of giving.