Authorities seized 122 fins from about 30 sharks, southeast from Golfito, off Punta Banco, in waters in the South Pacific.
The four arrested were all residents of Golfito.
Individuals were aboard the fishing vessel Yamauke, a long liner a small 12 meters long vessel, further the Yamauke fishing license had expired in May.
The police inspected the vessel and found the fins inside freezers.
This is the second case against shark finning in the South Pacific in June alone.
Since August last year there have been eight cases of shark fin poaching in Quepos, Puntarenas and Golfito.
Shark fin poaching in Costa Rica has been under the microscope of late; since Captain Paul Watson Sea Shepherd Conservation Society was charged with poach shark fins. Watson is currently fighting extradtion back to Costa Rica in Germany.
The Costa Rica News
San Jose Costa Rica