The police said the four - Bicky Gogoi, Bipul Gogoi, Saurabh Mahanta and Chandan Gogoi - were picked up from Titabor town in the district.
The miscreants had sent extortion notes to a local businessman Deben Hazarika demanding Rs four lakh.
They had claimed to be members of the ULFA and used fake pads and seals of the outfit.
Hazarika had filed an FIR in this regard last week.
Six mobile handsets and some pads and seals were recovered from them.