Easiest way to hide a button

If you have all this on one slide and are using different layers on that slide, you can't use "When timeline starts" to initiate the triggers. That's because the slide timeline will only start once. Going to a layer doesn't cause the timeline to restart. The good news is that you don't need variables. All you need is the states. Give a Visited state to each object that shows one of the layers. It doesn't even have to look different than the Normal state, in fact you may have reasons not to want it to. The important thing is that it have a Visited state. When an object with a Visited state is clicked, it automatically changes to Visited. Then you can create a trigger "Change state of [exit module] button to Normal when the state of ... is clicked". You will get a drop down of all the objects. Select those four that show the layers. This is the easiest method, but may not work well if the learner goes to other slides, then returns to this one. In that case, you need variables.  There was a time (I don't know if this has been changed), when it didn't work for more than about five objects, but with only four, you should be all right.