Hi,
Basically, they both provide builder tool, built-in page libraries/content blocks so that you can build your pages easily. The difference will come from the model they are based on, JA Builder is the old builder, which is built with SAAS model, its editors, tool are based on our server, whenever you edit a page, everything (tools, JS, CSS, etc...) will be pulled from our server. While T4 Page Builder is different, it will independent from server, once you install into your site, you can edit it normally. Of course, it still connects to our server when you import PRO page bundles, etc...
T4 Builder is not an upgrade of JA Builder so you can't migrate unfortunately.
You can find the more details here:
https://www.joomlart.com/ja-builder
https://www.joomlart.com/t4-builder/t4-joomla-page-builder
Regards