The bodies of the victims, a woman and three men, were found with their hands and legs bound in a warehouse they used as a base for their business in Krong Pinang district of Yala province.
Army spokesman Colonel Pramote Promin said they were Buddhists from the eastern Thai province of Rayong who regularly travelled to the Muslim-majority south to buy fruit.
"These people come to buy fruits in three southern provinces (Yala, Narathiwat and Pattani) like durian and longkong... they were familiar with the area and people," he said, adding a fifth trader escaped the attack.
Thailand's government has tried to stimulate the conflict-wracked southern economy by encouraging traders to buy fruit, rubber and other local produce for sale in the rest of the country.
But insurgents, who are fighting for greater autonomy for the southernmost provinces, perceive people with links to the government as collaborators and legitimate targets in their nine-year war.
On Febuary 1, militants opened fire on a truck-load of volunteers training farmers in Pattani province in rice-planting techniques, killing two and wounding a dozen others.
The militants believe outsiders seeking to boost the economy will "make them lose support and are exploiting resources from the area", Piya Kijthavorn, deputy secretary of Southern Border Province Administration Center (SBPAC) told AFP.
Near daily attacks -- including shootings, bombings and even beheadings -- mean violence is a part of life for many in Thailand's far south.
More than 5,300 people, both Buddhist and Muslim, have been killed since 2004, according to local conflict monitor Deep South Watch.