All the arrests, including that of ex-defence employee Magan Bahadur Singh, were made from the Garidhura area under Kurseong sub-division, said district police chief Kunal Agarwal here.
The ISI racket was busted following the interrogation of B K Sinha, an upper division clerk posted in the Indian Army's south-west command in Jaipur, who was arrested Friday.
"On being quizzed, Sinha confessed he had come in contact with an ISI agent through a retired Indian Army officer when he was posted in (West Bengal's) Siliguri between 2000 and 2011. It was also found that Sinha had flown to Kathmandu at least four time over the past two years and met ISI agents," said a police officer.
Sinha admitted to have passed on information regarding defence movement and handed over secret documents to the ISI operatives.
Based on the tip-offs given by Sinha about his associates, police picked up the four from Kurseong.
Besides Magan Bahadur, who lives in the Sukna area, the other three arrested were Gopal Biswakarma, Dhan Kumar Pradhan and Mizo Suriya. Biswakarma is a resident of Lalpul under Naxalbari police station, while Pradhan and Suriya are from Panighata undr Mirik police station, said Agarwal.
Army and central intelligence sources said ISI has been for long trying to use Siliguri as a corridor to increase its activities on the India-Nepal border. It has been making efforts to build sources in Siliguri, which is of strategic importance, because of its close proximity to the country's borders with Nepal, Bhutan and Bangladesh and as an entry point to the northeast.