The four - Lakha Odedara, Jayesh Kaccha, Arjun Parmar and Milan Parmar - were apprehended late last night when they were trying to flee from Porbander, they said.
Devani, an advocate by profession, was assaulted yesterday morning near his residence in Rajivnagar with sharp- edged weapon and sticks. He was seriously injured and admitted to the Civil Hospital.
Police suspect the incident was fallout of the RTI activist's campaign against illegal mining and construction in Porbander.
He has filed a writ petition in the Gujarat High Court and also complained to Chief Minister Narendra Modi against the construction of a hotel and a multi-storey building in the coastal city.
Devani has accused former BJP Minister Babhubhai Bokhiriya of orchestrating the attack, a charge denied by the latter.