The four were killed late yesterday after their vehicle was hit by a roadside bomb and militants opened fire, according to local police in Pattani - one of the three Muslim-majority provinces bordering Malaysia where the insurgency has raged for a decade.
Five others - including three soldiers - were injured in the attack, Khok Pho district police added.
The deaths were believed to be unrelated to today's general election which has seen scores of polling stations in Thailand closed by anti-government protesters.
"But I don't think the incident is related to the election, rather it is part of the (ongoing) unrest in the south," he added.
More From This Section
More than 5,900 people - the majority of them civilians - have been killed in ten years of conflict in the deep south, waged by shadowy insurgents seeking greater autonomy from Buddhist-majority Thailand.
The roots of the insurgency draw on long-standing anger at efforts by Thailand to assimilate ethnic Malay Muslims and at a perceived lack of respect for local language, religion and customs.
In an apparent breakthrough the previously secretive Barisan Revolusi Nasional (BRN) rebel group made open demands last year at peace discussions with Thailand.
But the talks have failed to progress as the BRN waits for the government to respond to its five-point peace proposal.
A source close to the discussions told AFP that more rebel groups are poised to join a new round of talks to be announced after today's election.
But other observers say lingering political discord in Bangkok is likely further to delay peace contacts.
The snap poll was called by embattled Thai Prime Minister Yingluck Shinawatra in a bid to end massive street rallies aimed at toppling her.