it could also be your anxiety about it that is making you think about it constantly. i know whenever i am anxious about anything i can't put the idea out of my brain, whether it is something i am interested in doing or not (i can't think of a personal example, but you know what i mean, right?)
i'm heterosexual, but i do the same thing you do; that is, i look at other girls' appearances and compare myself to them. everyone does this. it definitely doesn't mean you are sexually interested in the same gender.
i guess what i am saying is not to confuse anxiety with interest. you say you have no desire to be with guys. well, that's a big clue. if you are not physically attracted to them, then it is probably simply your anxiety that is turning a fleeting question into an obsessive thought.
however, if you can honestly assess yourself, and you figure out that there is a definite attraction, i say definitely explore it (as everyone else has suggested) and spare yourself and your girlfriend the misery of dragging out a relationship in which both partners are unfulfilled in some way.
another alternative may be that you are not homosexual, but you are subconsciously bored in your relationship (four years is a long-time, especially if you are young and have not dated a lot) and want to see what else is out there. if that is the case, i think you can be honest with your girlfriend about that, too. she's seems like a very caring and understanding person. maybe taking a break from the relationship to see what you really want would be the best thing for you right now.
either way, i hope it works out for you!
