Well you make four triggers.
Trigger1: name this trigger1
in the condition field put NOT ALIVE target1
you can make target1 whatever you want
in the activation field put
trigger1done=true
Trigger2= name this trigger2
in cond. put NOT ALIVE target2
activation: trigger2done=true
Trigger3= name this trigger3
condition: NOT ALIVE target3
activation: trigger3done=true
Trigger 4 = name this trigger4
condition: NOT ALIVE target4
activation: trigger4done=true
Now make a trigger 5 and in the condition field put
trigger1done; trigger2done; trigger3done; trigger4done
activation: "1" objstatus "DONE"; EndMission1
That should do it, the trigger1done might have a space like trigger1 done, but I'm not sure and some of the syntax isn't garunteed. Play around with it if it don't work.