Four mainland prisoners were injured in a brawl yesterday - the second clash among inmates within 14 days at the minimum-to-medium security Ma Po Ping jail on Lantau. The fight between two inmates and a group of another five prisoners broke out shortly before 1pm while the men were in a prison workshop. A row erupted between the two parties - all mainland illegal immigrants - and quickly degenerated into violence, with both sides attacking each other with plastic stools. Prison staff stopped the fight but four detainees, aged from 22 to 25, were reported to have sustained head injuries. Three were sent to Queen Elizabeth Hospital by helicopter and police launches.