Fixed this. It only took about 25 steps, which in SL projects, isn't that many.
Add a shape (rectangle) that completely covers the four cards. Set the transparency of the shape to 100%. This will effectively keep anyone from clicking any of the cards.
Next, go into shape states and set the shape to initially: hidden. This allows the user to click any card, as long as the shape is hidden.
Then, add a trigger on every card. _change state of rectangle1 to normal when user clicks card1_ This means that as soon as the user clicks any card, the shape "appears" (even though still invisible) and blocks the user from clicking any other card.
You will need to repeat these steps on every layer of the main game slide (5 layers). Make sure that on each layer, the triggers are pointing to the shape on that layer, not the base layer.