The incident occurred when driver of the bus, in a bid to overtake a tempo, lost control, police said.
Some of the passengers were travelling on the roof of the ill-fated bus which was on its way to Gadraroad town, they said.
Tara, Roshan, Satram and Chenaram died on the spot, they said.
Out of the 26 injured, 15 were given primary treatment and remaining nine were being treated at Barmer government hospital, the police said.