Your 4WD warning light is on because your vehicle's computer has detected an issue in the four-wheel drive system. This could be triggered by a temporary glitch, such as a brief signal interruption, or by a real component failure, such as a defective sensor, faulty wiring, a stuck 4WD actuator, or a problem with the transfer case. Sometimes the light appears simply because the 4WD system didn't complete a mode shift properly, especially if you switched between 2WD and 4WD while the vehicle was moving too fast. Temporary faults will automatically reset when the vehicle is turned off for a brief period. If the warning light returns, and stays on, bring your vehicle in for a diagnostic scan so the exact fault code can be identified and addressed.