It looks like by the time you read this, you may have already made your choice. However if you haven't, my advice is this:
If you've been in love with her for four years, and have spent lots of time with her (you probly have) and nothing has happened yet....than chances are that nothing should happen. You two are FRIENDS. Over the years, weather you know it or not, you have likely sent out dozens apon dozens of subtle gestures that you like/love her. It may be the way you look at her, your movements...whatever. You know what it is. No matter how subtle these gestures are, I guarantee that over four years she is going to pick up on them. If she wanted a relationship with you, she would have initiated something with you by now. Because she may not know that you are in love with her, but I'm sure that she's aware on some level that you WANT HER.
Don't waste your time pursuing what will just hurt you really bad. And let me tell you, I've been through this myself. It wasn't me that got hurt, but the guy (I'm a chick) that was in love with me did get hurt. I didn't mean to do it, but I just didn't feel the same way. We were friends for too long. It's been 6 years now, and I can still see the hurt in his eyes. We're still friends.
I hope you take my advice and get past it.