I'd love to help, but I'm only 20, without a girlfriend anyhoo. So this is just stuff I read and agreed with. Perhaps as consideration.
As far as I know, relations change over time, and at some point the sexual drive of one or both partners can just give out. It's a new stage of a relation. However, you should find out why your girlfriend lost that drive. First you have to honestly answer the question (to yourself) whether it existed honestly on her part at all. It probably was, and if so, you'll have a good chance of bringing it back.
With the little experience I have, and the small knowledge of your situation, I can think of few reasons. The most likely one to me is: After 4 years, she is your girlfriend, not your wife or fiancee. If she feels that your relation will never exceed the level of lovers, I can imagine that that works as a large blocking mechanism.
Well, that doesn't yet relate much to your military service. Arguably, those weekends added a compulsary tinge to bedsharing in the weekends? Is it perhaps that she has got the feeling that she is the fuckfriend (excuse the rude way of expressing)? I really know too little about it. However, if we're at all to make a good evaluation of the situation, more info is necessary.