I JUST got over mono as well. Luckily it only lasted a month, and my spleen didn't enlarge (like it sometimes can), so I stepped right back into the gym. It's definitely frustrating because I've lost some strength.
It's been four months since you were able to go to the gym? My advice would be to take it really slow. Low weights, maybe higher reps. That's more than enough time for your body to revert back to not being "gym-ready", so you'll want to be careful the first couple weeks going back. And get ready for some extreme soreness, too. Hopefully your body will be readjusted to the gym within a few weeks of taking it slow.
I'm not entirely sure, but I don't think you can get a "relapse" of mono unless you become severely immuno-compromised (such as getting a bone marrow transplant, etc.). It's true the virus stays with you, lying dormant, but regular gym stress isn't going to make you have a relapse (unless you still have mono, in which case you haven't gotten over it in the first place).
Mono is the WORST...at least now we're immune, right?