The 4wd switch works off an actuator, 2 of our 3 teryx's have this issue as well. Lube the cables really well, check the vacuum lines coming and going from the actuator, and worse case, reach under and manually shift it out of 4wd from the diff. The 4wd lever is on the passenger side of the bike and the difflock is on the driver side. All 3 of ours have had to have new difflock cables installed because they like to get plugged up with mud and seize. We grease them very heavily now before installing them to prevent the issues. The actuator is under the main engine plastic right by the passengers calf at the front of the belt box mounted to the seat bracing.