The vehicle, on its way to Jammu from Srinagar, belonged to Justice M K Hanjura of Jammu and Kashmir High Court, a police spokesman said.
The judge was not travelling in the vehicle as he was scheduled to travel by air, he said.
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The vehicle fell into the 200-metre gorge near Kela Morh, 7 km from Ramban town, around 9.45 am.
Rescuers immediately swung into action and rushed the four injured to a hospital, where one of them was declared brought dead and two others succumbed to their injuries, Senior Superintendent of Police, Ramban, Mohan Lal, said.
The deceased were identified as personal security officer (PSO) of the judge, constable Ajit Singh (34) of 175th battalion CRPF, driver Vinod Kumar (35) and cook Hukum Chand (39), the SSP said.
Another PSO, also a CRPF jawan, was injured in the accident and is undergoing treatment, he added.