I'm doing it right now.
Although we're about to upgrade our production server to 4.0. But currently, we're running a 4.0 test BES and a 3.6 production BES. You'll use the same BES administrator service account. Obviously, you'll designate a different database (you may want to put it local on the BES if its in test, unless you want to replicate your current production environment (although I'm not sure how you have that setup either)).
Anyhow, you'll be fine. No conflicts whatsoever.