The explosion hit a checkpoint in Khok Pho district of Pattani province yesterday evening, killing four Buddhist men aged between 43 and 54.
"Four village defence volunteers were killed and four wounded, who are still hospitalised," local police Colonel Uthai Thipsopa told AFP from the blast site.
In recent years Thai authorities have armed village volunteers to protect their hamlets from frequent gun and bomb attacks by Malay Muslim rebels seeking greater autonomy.
The Muslim-majority south was an ethnic Malay sultanate until Thailand annexed it more than a century ago and resentment has long festered over forced assimilation schemes.
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The rebels routinely launch attacks on security forces and have terrorised civilians seen as sympathetic to the Buddhist-majority state - both Buddhist and Muslim.
Authorities also stand accused of perpetrating severe rights abuses, including arbitrary arrests and extrajudicial killings.
Rights groups have previously expressed fears that arming local volunteers promotes vigilantism in a region already scored by suspicion and impunity for civilian deaths.