I don't know how the video describes the installation, you can try ask the author of the video for guidance.
If you'd like to the try the on-going development branch for PI4 managed by RetroPie, then you'd need to perform a manual installation using a Raspbian Lite image and the fkms_rpi4 branch of the RetroPie repository. You'll need to use the command line to do that, so if you're not familiar or comfortable doing that, then you should just wait until a Pi4 supported image will be released.
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