So we are using Reporting V3 API in our custom dashboard to retrieve custom reports from multiple GA properties. Now some of our customers have GA4 properties which doesn't show up in properties list. Question is, to support both versions (GA3 and GA4) the only alternative now is to use Reporting V3 and Data API for different properties or did we miss something and there's one API which can be used for both GA versions?
There are two versions of Google analytics now Universal analytics and GA4 they are completely different and have different APIs designed for each one.
Google analytics reporting api allows you to access data from Universal analytics accounts.
Google Analytics GA4 is a completely different system and is not supported by the reporting api to access data behind a GA4 account you will need to use the Google analytics data api
there is also the Google analytics admin API which is the equivalent for the Management api for GA4. so if you want to be able to list a customers GA4 accounts you will need to use that api as well.
Note: The admin api is currently in alpha, and the data api is currently in beta. Take that into account before developing production applications using these apis.