The latest in a string of rebel attacks took place near a military checkpoint in the centre of Pattani town, 800 kilometres south of Bangkok.
Two paramilitary defence volunteers and two civilians were killed in the blast, according to military officials.
Two other bombs exploded, wounding one civilian, but security forces reportedly managed to disarm the others.
Army commander General Prayuth Chan-ocha speculated the attacks were retaliation for a clash Wednesday in neighbouring Narathiwat province that left 16 rebels dead but no government casualties.
Five soldiers were killed last Sunday by a roadside bomb in Yala province.//DPA