The accident occurred on the Cha-am-Pran Buri bypass in Hua Hin district of Prachuap Khiri Khan province.
All of the passengers in the pick-up truck were supporters of the People's Democratic Reform Committee (PDRC) returning to Surat Thani province after taking part in the protest at the Lumpini Park rally site.
Somjai Chuchuaysuwan, one of the injured, said their pickup truck was hit and forced off the road by a 10-wheel truck. The pickup crashed into a tree on the median strip.
Killed were Amporn Yangyuen, Wuthisak Kaewbuathong, Thavorn Chanthuros and Saowalak Yodpromthong.
The injured survivors are Mr Somjai, an assistant village chief in Phra Saeng district, Rithikrai Momit, Nopporn Urairote, a member of Bang Sawan tambon administration organisation (TAO) in Phra Saeng district, Wirat Manmuang, Kosol Kiewtang and Somporn Anupat, an assistant vilage chief in tambon Bang Sawan, police said.