It’s possible to send data into both UA and GA4. That way there is historical data for comparison after the UA data is deleted next year.
I put the UA-gtag.js code snippet in <head> and added one more line to send data into GA4:
gtag('config', 'UA-********-**', { "anonymize_ip": true });
gtag('config', 'G-*********', { "anonymize_ip": true );
Does anyone know what additional configuration might be required? It seems to be working here, but my GA configuration is simple.