Setting up Conditional States and Triggers

Alternatively, you can use four True/False variables, each initially set to False (e.g., video1Complete, video2Complete, etc.), and an additional number variable called videosCount set to 0.
When any video on any layer completes, run the following two triggers:

1. Add 1 to videosCount when the video completes, if videoXComplete is False.
2. Set videoXComplete to True when the video completes, if videoXComplete is False.

Example:

This ensures videosCount only increases once per video and gives you a clean way to track progress. 
On the base layer, create a trigger: Change the state of the Next to Normal when the variable videosCount changes, if videosCount is greater than or equal to 4. See the attached example.