maybe she thinks that she's got you in the palm of her hands...
do the right thing for yourself, now is the good time to be selfish and slowly back down from the relationship. If she really loves you then she would find a way to make things right again. If she doesn't, then it shouldn't hurt that much since you've already started to create a distance. Be honest to her as well, tell her what you really feel and what you think about the situation, and what you are planning to do. Expect the worst to happen...
4 years is great, but even 1 year of hiding the truth does far more bearing than those 4 years.