The bomber targeted a gathering of Taliban fighters who were celebrating a three-day truce.
The rollback of territory under its control has not destroyed ISIS, but caused it to revert to its roots as an insurgency.
Turkey has repeatedly called on the U.S. to stop backing the YPG, which it considers to be terrorists.
Two Marine rifle companies supported Special Operations Command units operating in northern Syria between January and April.
The U.S. will intensify combat against the ISIS affiliate in Afghanistan during Kabul's halt to attacks on the Taliban.
U.S and coalition forces have increased strikes against ISIS fighters in Syria by more than 300 percent since March.
There are 329 parliament seats at stake, with nearly 7,000 candidates from dozens of political alliances.
The four troops killed in the Niger fought with honor, according to a Pentagon account.
Iraqi forces in coordination with U.S.-backed Syrian forces have captured five senior Islamic State group leaders.
U.S. and coalition forces have increased airstrikes and artillery fire against ISIS fighters in support of Operation Roundup.