Police said the accident happened at 2 am on Ngao-Song Khet Road in Ngoa district.
Police said the trailer driver apparently dozed off and crashed into the pickup on the opposite lane.
The pickup driver Sayam Pinthong, 31, died at the scene along with three Myanmar workers. Another Myanmar worker died at the Ngao hospital. Four injured Myanmar workers were later forwarded from the Ngao Hospital to the Lampang Hospital.
The trailer driver, Somkid Rungphaophan, 37, was injured and arrested by police.