Need Help: Enabling Next Button After Visiting 4 Slide Layers

For enabling Next after layers are viewed, I typically use a T/F variable to track whether each layer has been viewed. The triggers on each layer would look something like this: 

Another option: If you present the layers as Dialog layers, the user can't interact with anything else until they close the layer. So you could enable the Next button based on the layer-opening buttons being in the Visited state. 

Both of those options are demonstrated in the file attached to this post: 

TIP: Controlling the NEXT Button 101 | E-Learning Heroes 

FYI: Based on your description, it sounds like you want to automatically show a 5th layer after the main 4 layers are viewed. I recommend against showing that automatically, because that might confuse the learner if the content suddenly changes. They should be in control. Instead, you could program it so the button to open the 5th layer appears/is enabled after the other 4 layers are viewed.