GA4 vs Universal Analytics: What's the Difference?

Universal Analytics (UA) measured activity as sessions built from hits - pageviews, events, and transactions bucketed into a session with metrics like bounce rate calculated against that structure. It was built for a web-only, single-device world.

GA4 measures everything - a pageview, a click, a purchase - as an event with parameters attached to it, with no structural distinction between a "pageview hit" and a "custom event." The same event model works across web and app, which is why GA4 can track a single user moving between a mobile app and a website in one property.

The two aren't just different dashboards on the same data - they're different measurement models, which is why metrics don't map cleanly between them. Universal Analytics stopped processing new data in July 2023, so this isn't a choice between two active options; every site still on UA reporting is looking at historical data only. Migrating isn't a settings change - it's rebuilding tracking around events and parameters instead of sessions and hits, which is why a proper GA4 migration audits what's actually being measured rather than just installing a new tracking snippet.