There are actually 5 disc brakes on these machines. 1 for each wheel and 1 on the rear driveshaft for the parking brake.
I've read several complaints of the parking brake one being too tight from the factory so I would check the cable and make sure there's a little slack when the parking brake is released.
If that is good I would jack up each corner Individually and spin the wheel to listen for noise and feel for dragging. That should help pinpoint where the issue is.