Need some info about mononucleosis?

Well, I think given that information it would be difficult for even a doctor to give you a diagnosis. Mono lasting 4 years- probably not, but there are hundreds of other things it could be: Epstein Barr, Chronic Fatigue, Lyme disease, vitamin deficiency, not enough sleep, HIV, TB, chronic strep throat, a drinking problem. Maybe he liked the attention and concern he got when he did have Mono. Maybe he likes having a built in excuse to not go out much and doesn't want to be badgered about why. Maybe it seems like a simple answer when in reality he has something like non-hodgkins lymphoma or some other cancer. When you tell people you have cancer, people treat you differently and feel sorry for you- and they all want to know very intimate and insensitive details (such as things like "how long do you have to live").

Why don't you ask him? If he doesn't tell you, it means he doesn't want to tell you- for whatever reason. If you are a good friend with him, ask about how his life is going in general. Maybe feeling like someone cares will get him to open up more, maybe not. But respect his decision either way.