Four Thai army rangers were killed Friday when a roadside bomb tore through their truck as they were securing a road for teachers, an army official said, the latest carnage in an insurgency roiling the kingdom's far south.The Muslim-majority region that borders Malaysia has seethed with violence for over a decade as ethnic Malay insurgents battle the Buddhist-majority state for more autonomy.More than 6,800 people have died – the majority civilians – in a conflict marked by weekly bombin