Well, I'm not competent with GUI, but I think you just need to create a trigger that says that whenever one of the required units dies, say, a Demonic Summoner, that you add one to the variable DS, then you run a check that the variable DS is equal to, say, four. If it is equal to four, then turn off the trigger and complete the quest. If it isn't, do nothing.
I think that is how it should go. As I said, I'm not competent with GUI.
If you're making a trigger in JASS or Wurst, I am unable to help you.