The bodies of the four were discovered Sunday shot in the head and with their hands tied behind their backs in Vavuniya district, the official said.
"Four people were found shot dead at Tavasakulam in Vavuniya," said a police official in Vavuniya, 260 kilometres (160 miles) north of Colombo.
"An investigation is under way," he said adding that it was unclear who was behind the killings.
The four were killed in the area where security forces and the rebel Liberation Tigers of Tamil Eelam (LTTE) traded artillery and mortar fire on Saturday, officials said.
The two sides both claimed to have inflicted heavy casualties in the weekend fighting with the military saying at least 16 government soldiers and three rebels were killed.
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The Sri Lankan defence ministry cited different figures, saying at 10 soldiers were killed and 34 wounded.
The ministry said "a large number" of Tigers were also killed.
Claims by either side cannot be independently verified.
The latest fight