HiI have a Nissan Patrol and while driving on...

Dear Sir
Welcome, and thank you for trusting us
Service 4WD light is on, there may be an issue with the system
illuminated if there is a malfunction in the 4WD system. The light will also come on if the front wheel or tire diameters are different from those on the rear. In either case, 

1. check the oil .
2. check the sensor 

and I can help you more if you check the car on OBD II  device and send to me the codes 
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Kind Regards
Engineer mohammad