Moving to Google Analytic 4 (GA4), there is no direct equivalent setDomainName as in Universal Google Analytics. The new GA4 has a unique data model and configuration settings. Also, the gtag() function does not have a _setDomainName method that’s why you’re getting an error.

In GA4, you can configure your domain at a property level through the data streams. You need to visit the GA4 property settings essentially. Within settings, select ‘Data Streams’ (either Web, Android App, or iOS app) and you can set up your domains. This makes it unnecessary to manually set the domain in your tracking code.

For more advanced domain configuration needs, you might need to consider Google Tag Manager or custom tracking parameters. It’s better to refer to the GA4 official documentation to understand how to properly configure your properties for multiple or specific domains.