You're on the right track with Conditional Actions. What you need is a wide trigger, and then a condition to narrow the scope to the state you are interested in (when ANY of the Stacks are hidden, check to see that they are ALL currently hidden). In this situation, because it's all on the same page, you can use the Stacks for both your triggers and your condition.
Here's a quick video - https://share.smartbuilder.com/public/support/show-stack-when-other-stacks-hidden.mp4
When the first Stack is hidden, the action starts, but the condition fails, so nothing happens. When the second Stack is hidden, the action starts, the condition is met, and so the action inside the condition (show Stack 4) executes.
If you didn't have the Stacks right there to query their visibility, you could also use variables to keep track of which items were clicked.
Hope that helps!