Three of the Americans died in southern Afghanistan on Sunday, the military force said in a statement. Two of them were killed in a bomb attack and the third in a separate firefight.
The military said the fourth U.S. service member died in the east Monday in a bomb explosion.
The deaths bring the number of Americans killed in Afghanistan in November to 15. October was the deadliest month for U.S. troops in the eight-year war, with 58 dead.
Meanwhile, Afghan officials said three Afghan army soldiers were killed by a roadside bomb in southern Helmand province.
The Interior Ministry said the bombing occurred on Sunday in Musa Qala district. It did not give further details and it was not clear if there was any connection between the Afghan deaths and the American deaths.
In Brussels, NATO said Monday it wants allied nations to commit more forces to Afghanistan ahead of a U.S. decision on whether to send more troops.
NATO Secretary-General Anders Fogh Rasmussen is in the midst of intense consultations on getting more troops, equipment, funding and other resources for the newly established NATO Training Mission, spokesman James Appathurai said.
NATO currently has about 71,000 troops in Afghanistan, nearly half of them American. The U.S. military also has another 36,000 soldiers in Afghanistan who serve outside NATO under independent command.
The Obama administration is considering military proposals to send 10,000 to 40,000 additional U.S. troops to Afghanistan next year.