Google Universal Analytics (= Google Analytics 3) is the replacement of the classic Analytics and has been around since late 2012. The age and functionalities of GA3 are starting to weigh on the tracking capabilities. That’s why Google wants to make the switch to the newer version.
The 3rd party cookie issues in Europe may also have an influence on this. The new Google Analytics 4 is a cookie-free method for gathering and measuring website visitors and other relevant data.
The biggest difference is in the measurement method. For example, Google Analytics 4 does not use page views, transactions, social hits..., but hits are seen as an event.
In addition, the Google Analytics 4 dashboard has a completely new interface, making it easier to find data.
The path Google is following with the change to GA4 is rather atypical. In previous versions, you were not required to upgrade, and you always had the option of accessing your old data. With this switch, that is not the case anymore and you will lose almost all historical data from Universal Analytics as of 2024.