Police said the blast occurred at the Chatchai Chonthan Rungsawat dockyard in Bang Sai district at 1:50 pm.
The blast was apparently sparked by a gas welder, police said.
The three Thai workers were identified as Sawieng Wejsawat, Surin Subsueb, and Banchong Pithakul. The Cambodian worker was identified only as Chu.