You need to set up your dojoConfig like in the video, before the CDN script tag. You have it after the CDN script tag, so it doesn't get picked up.
<script>
window.dojoConfig = {
async: true,
packages: [
{
name: "esri4",
location: "https://js.arcgis.com/4.8/esri"
}
],
has: {
"dojo-preload-i18n-Api": false
}
}
</script>
<script src="https://js.arcgis.com/3.25/"></script>
This way you can reference 3x and 4x correctly.
Here is the demo from the video.
JS Bin - Collaborative JavaScript Debugging
There are limitations, but you can do some stuff.