The four (men/ guards/ people 或者什麼都没有也行) are guarding four doors respectively.
或者个人觉得更顺一点的:
The four are each guarding a door
又或者:The four doors are guarded by four (men/ guard) respectively.
The four doors are each guarded by a (men/ guard).(最後这个两个大概最好)
楼上:respectively 应该是不能用单数a door 的.(除非是每四个人看守一道门) -----虽然我也不太确定