Hello,
It's very simple to add GA4 to a mojoPortal site. All you need to do is open the layout.master for your skin and add the code you referenced to the bottom of the <head> section of the html.
If you look at the source on the i7MEDIA website, you'll see our GA4 tag just above the </head>
We're finishing up on our work to get the pgsql, sqlite, and mysql database layers in sync with our mssql database layer. Once we have done that, we'll be able to make a public release which includes a feature to add scripts to the site without editing the layout.master.
Thanks,
Joe