They were discovered between May 26 and June 2 in the Bay of Puck, according to local police spokesman Krzysztof Kusmierczyk, who said the deaths were caused by a human and come with a three-year jail term.
"The first two seals were found tied together with a cord that had a brick at the end," he told AFP.
The third seal suffered blows from a hard object while the last one was found with its stomach cut open, he said.
A century ago there were 100,000 grey seals in the Baltic sea but by the 1980s there were only between 2,000 to 3,000, leading Poland to place them under protection.
Today there are around 20,000 seals in the Baltic sea, mainly on the coasts of Estonia, Finland and Sweden.
The mammals have almost entirely disappeared from the southern part of the sea. Notable causes include chemical pollution, which has rendered the females sterile, and seal hunting, which Poland has since banned.
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