But hold on.

The ac system is a sealed unit, but if its already low on R-134a and its only 4-5 years old, the freon has to have gone somewhere right?

Normally they dont just get low. Its a 2.2ish lbs system and the compressor will still run with roughly 1 to 1.3 lbs of freon in there, so where did the other amount go?

Personally, I'd take it somewhere and just have them do the simple AC check, then recharge it if everything is ok.