By popular demand, Google Analytics 4 is up and running for Site Builder! 

Sign up and Create a Google Analytics 4 Account, and then Create a GA4 Property for your site.

Once you have a Google Analytics 4 property number, you can go to your Site Builder site, and add the GA4 Property under the Site Information and Appearance section, toward the bottom of the page.

You can continue using your original Universal Analytics Property until Google shuts it down later this year.  Your older UA Property will have historical data that...