Helmand- In the Gereshk district of Helmand province, three British troops were struck by a blast and pronounced dead at the scene.
The 4th a Royal Marine was hit by enemy fire near Sangin and died in a UK hospital, The Ministry of Defence (MOD) said.
The death toll of UK servicemen and women killed in Afghanistan, since 2001, reached 149.
It was the worst loss of life in a single incident since last December, according to reports.