I'm hoping you're particularly perceptive, because by the time most guys notice something, the girl has mentally left the relationship weeks/months ago.
4 years hey? If you're over 25, then she could be annoyed that you haven't asked her to marry her yet. Has that come up in any conversations in the last year or so? If this is the issue and you want to keep her around, then there's one solution for you.
Another one is to just become that cool, fun guy again that she first met (I'm not saying you aren't, but she's just used to you now) and mix things up a bit. You might want to try doing some different things, preferably active stuff like kayaking or mountain biking. Make sure you invite her along, but also make it clear that you'll do these things with or without her.
Good luck