Steps

1. Go to the Google Analytics account
2. https://analytics.google.com/analytics/web
3. Go to all properties (so site in GA3, old system)
4. > A message similar to the following will be in a red banner at the top "This property has stopped processing data. To continue measuring the performance of your website, configure a Google Analytics 4 (GA4) property."
5. Let yourself be guided by clicking on the “Finish GA4 configuration” button next to the message from the previous point
6. Click on the “Create your property” button
7. Click on the “Finish GA4 configuration” button
8. Click on the “Access your GA4 property” button
9. > A new entry will be available in the properties
10. In the left menu, the "Data flow" subsection will contain the new identifier (c.f.: G-XXXXXXXX) and the new HTML code to put in the tag of the pages to follow (or your head file. html)
11. <!-- Google tag (gtag.js) -->
12. " target="_blank" style="pointer-events: all;">https://www.googletagmanager.com/gtag/js?id=G-XXXXXXXX">
13. <script>
14. window.dataLayer = window.dataLayer || [];
15. function gtag(){dataLayer.push(arguments);}
16. gtag('js', new Date());
17. gtag('config', 'G-XXXXXXXX');
18. </script>