The accused were on the way to Bhilwara in Rajasthan from Jharkhand, he said.
There was an information that an inter-state smuggling racket was operational. A gang operating from Bhilwara was put under surveillance, the official said.
ATS sleuths trailed them from Jharkhand and arrested them in Simliya village of Kota district last night with 50 kg opium, ATS DIG Vikas Kumar said.
The accused were identified as Hari Lal, Babu Lal, Ram Lal and Narayan Jat. All are residents of Bhilwara district, he said.