What is the difference between Universal Analytics and GA4?

They use fundamentally different data models — GA4 is not an upgrade of Universal Analytics but a replacement built on a different foundation.

• The measurement model. Universal Analytics was session-based: hits grouped into sessions, with pageviews, events, and transactions as distinct hit types. GA4 is event-based: everything is an event with parameters, including pageviews. This is the core difference and everything else follows from it.
• Cross-platform. GA4 measures web and app in one property; Universal Analytics could not.
• Identity. GA4 uses a blended approach — user ID, Google signals, device ID, and modelling — rather than relying solely on cookies.
• Metrics changed. Bounce rate was replaced by engagement rate, its inverse: a session is engaged if it lasts over ten seconds, has a conversion, or has two or more pageviews. This is a genuinely better measure, because a user reading one page for three minutes was counted as a bounce before.
• Reporting. Fewer standard reports, with the Explore section providing free-form analysis, funnels, and path exploration.
• BigQuery export is free in GA4, which was a paid 360 feature before. This is a major change for anyone doing serious analysis.

Note: Universal Analytics stopped processing data in July 2023, so GA4 is the only option. Historical UA data does not migrate, which is why year-over-year comparison across the transition is genuinely difficult.