Just ask her why she doesn't wanna have sex with you?
Why would you want a 4-year relationship to go down the shitter over a nut? Are you still intimate on the couch? Do you guys laugh and have fun? If it's ALL bad, that's one thing, but if it's just sex, she's probably just bored of you.
It happens man. They call it the 3-year glitch and the 7-year itch. The timing doesn't matter. The point is that you are in a rut.
It's super immature to just be like, "fuck, 60 days have gone by and it's all over." That's insulting to anyone who travels abroad for work (military?) How do you think those guys handle/cope?
By the 4-year mark, I'd venture to say you are in love with the woman. If not, what the fuck are you even doing? Being that you are in love, you have to compromise. Jumping to conclusions and overthinking helps no one. You have a strong option here, and that is communication. Make dinner, sit her down, and let her know what's up. Be receptive and approachable.
I'm not saying to give up your man card here, but fuck man. This is an easy fix. Talk first like GLY said. Find out the root of the issue. Did you stop taking care of yourself? Is she depressed about something? There's 10,000 reasons why this could be happening, and you're giving yourself that as an out?
Grow up bud. If no one else is willing to give it to ya straight, I always will. ;P