What is the difference between GA and GA4 from a data perspective?

The GA3 data model is based on sessions and pageviews, while the GA4 data model is based on events and parameters. This means that both GA3 and GA4 can collect, process, and report the same data differently.

Additionally, some aspects of session counting in Google Analytics 4 differ from Universal Analytics. For example, in Universal Analytics, a new campaign will start a new session regardless of activity. In Google Analytics 4, a new campaign does not begin a new session. This may lead to lower session counts in your Google Analytics 4 property.

Google Analytics Supports mentions the main differences in the below article:

https://support.google.com/analytics/answer/9964640?hl=en#zippy=%2Cin-this-article