Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
Sure. AirTrain takes about 6 minutes between T8 and T4, in either direction. Just make sure that you board the "All Terminals" AirTrain when going from T8 to T4, or you'll be in for a very long ride.
From T4 back to T8, take the first AirTrain to arrive, regardless of posted destination or direction of travel.