My question is do these stacked column popups that you can create in the new Map Viewer required the 4.X JS API to display or can they be displayed in Web AppBuilder using the 3.X JS API.
They can not be displayed in WAB, or any map on the 3.x API. Once you configure your popup to have columns, or any other new capability, opening that map in a 3.x map will only give you a portion of the popup.
If there's no WAB-specific widgets holding you back, you might look into Experience Builder for getting a 4.x compatible web app. But we've got a few maps/apps that, at least for now, have to stay in the 3.x tools, and so we have to put in extra work to create "fancy" popups.