4．如图，三棱台ABC−A1B1C1ABC-A_{1}B_{1}C_{1}ABC−A1B1C1中，BC=ACBC=ACBC=AC，现在以下四项中选择一个，可以证明AA1=BB1AA_{1}=BB_{1}AA1=BB1的条件有：①CC1⊥ABCC_{1}\bot ABCC1⊥AB；②A1B1=A1C1A_{1}B_{1}=A_{1}C_{1}A1B1=A1C1；③∠C1CA=∠C1CB\angle C_{1}CA=\angle C_{1}CB∠C1CA=∠C1CB；④∠A1AC=∠B1BC\angle A_{1}AC=\angle B_{1}BC∠A1AC=∠B1BC．((( )))
