Captain Iuliu-Vasile Unguras, 32, who was leading the patrol mission formed of four Humvees on the A1 highway linking Kandahar to Kabul, was killed in the blast.
The four injured were immediately taken to military hospitals in Lagman and Kandahar. The ministry said all four are medically stable.
Captain Unguras, the third Romanian to be killed in Afghanistan this year, was married and had two children.
Another Romanian officer was killed Friday morning in Afghanistan, about 80 kilometers south-west of Kabul, while fighting insurgents alongside a U.S. mission. He was 33, married, with a child.
Eleven Romanians were killed in Afghanistan since 2003, when Romania started participating to NATO missions in the country.
Romania currently has 956 troops in Afghanistan, according to the Defense Ministry.