The car bombings in two southern cities on Saturday killed 15 people and wounded hundreds more in the deadliest attacks in the region in recent years.
Three Muslim men were detained on Wednesday evening at a house in Rueso district of Narathiwat province in an operation involving 50 police and paramilitaries, authorities said.
A stolen pick-up truck which was used in the attack in the city of Yala was earlier seen in front of the house of one of the suspects, said Colonel Satarnfa Wamasing of Rueso district police.
He said the licence plate of the truck was also put on a stolen car used in an attack on a hotel in the city of Hat Yai on Saturday.
"They still deny it but we have evidence that they met to plot the attacks," Satarnfa told AFP on telephone, adding that police were investigating records of their phone calls.
"There are more people involved but I can't say how many," he added.
One of the suspects is a Muslim cleric with nine outstanding arrest warrants, Satarnfa said.
The fourth suspect, who was arrested at his home in Yala province on Wednesday, was filmed by a security camera riding a motorcycle to pick up the driver of one of the vehicles used in the twin blasts in Yala, police said.
He also denies involvement.
The attacks marked an apparent escalation of a shadowy insurgency, without clearly stated aims, that has claimed thousands of lives since 2004.
The near-daily bomb or gun attacks are indiscriminate, targeting both soldiers and civilians, Buddhists and Muslims.
A state of emergency is in force in the worst-affected parts of the region, which rights campaigners say in effect gives the tens of thousands of military troops based in the area legal immunity and fuels rights abuses.