The victims, who were all men, were travelling in a pick-up truck late yesterday when they were ambushed by gunmen in the Nongchik district of Pattani province, police said.
"The initial motive of the killings is linked to the insurgency," Colonel Yukol Prasatnanont, superintendent of Nongchik police said, without giving details.
He added a fifth man was missing, while two others sustained minor wounds in the attack.
In a separate incident, a 49-year-old Muslim villager was shot dead at his home late yesterday evening in Pattani's Kapho district.
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The grinding conflict shows no sign of easing despite the talks, hosted by Malaysia, between the Thai authorities and one of a network of rebel groups.
A fresh round of discussions had been slated for December but Thai authorities said they may be postponed amid political unrest in Bangkok, where anti-government protesters have clashed with police.
"We will try to have the talks this month, but if we cannot we will postpone," said Paradorn Pattanatabut, head of Thailand's National Security Council.
Experts also say a workable roadmap to peace remains far off as Thailand is yet to formulate its response to demands made by rebels at the talks - including to recognise them as a liberation movement.