from Slack msg: The TLDR is:
- You can continue to leverage your original GA Universal tag to stream data into GA4
- It's definitely smart to begin setting up GA4 and have the data come in asap, since it's not possible to retroactively load data into GA.
- GA classic will not be realistically deprecated for a very long time (given its install base and enormous amount of active users around the world)- GA4 still lacks a number of key features GA classic provides.
- GA4 still lacks sufficient API support for us to properly ingest the data. This is a dynamic situation and will change quickly as ingestion technology catches up to GA4.
For likely the rest of the year, (and beyond) we'll want to continue ingesting GA classic data. In the meantime, it's a good idea to have GA4 setup ASAP. We recommend simply leveraging your existing GA tag. It can also be setup independently. Hope this helps!