Layering

Hi 

I am trying to build a course that has four videos. Initially, only the first video is being played and the rest all are locked. However, as and when the video plays, the lock disappears. For example, if Video 2 is being played. Then only Video 1 and Video are open and Video 3 and Video 4 are locked. But, if all videos are played at least once, then lock disappears. We have to create this template on one slide. However, we need to ensure that the developer adds the video just four times and not all on the layers. Please let us know if it is possible. I have attached the file for reference.