The Taliban claimed responsibility for the attack.
The four Americans were members of the International Security Assistance Force serving in eastern Afghanistan, the ISAF said.
The U.S. also handed over security responsibilities to the Afghan government Tuesday, even as violence escalated throughout the conflict-ridden nation.
Afghan President Hamid Karzai said Tuesday he would suspend security talks with the U.S. in protest of the outreach to the Taliban.
Rosie O'Donnell unveils nearly 50-pound weight loss
Members of Congress to keep receiving porn magazine