The most important part is making sure your room/screen with the pickup and your room/screen with the door are set to the same screen type and that your pickup item is set as a monster type pickup. Your door also needs to be a tile not a monster and set as tile type All Pickups Lock.
I also had a major issue when I was trying to do it where the door wouldn't open, but I was trying to do something advanced and put more then one type of monster in the door room and the pickup room which the engine didn't like. It would switch to the triggered monsters but the door wouldn't open. So in your pickup room make all 4 monsters the key but place just one in the room, and in your door room make all day monsters the same and all triggered monsters the same and see if that helps.
After you know it's working, then you can experiment.