BEIRUT — An airstrike in northern Syria killed four medics responding to an earlier bombing raid, a relief group said Wednesday, as the U.N. announced it would resume aid deliveries suspended after an attack on a convoy two days ago that killed 20 people. The escalation of violence against humanitarian workers has all but destroyed a cease-fire that took effect on September 12, and has stoked tensions between the truce’s architects, the U.S.