GA4 is Google's latest version of Google Analytics that allows combining mobile app and website usage data. It uses an event-based data model instead of pages and sessions. While GA4 provides improved cross-device tracking and engagement metrics, it still lacks robust ecommerce reporting and attribution features found in GA3. Key differences between GA4 and GA3 include different measurement models, tracking IDs, custom dimensions/metrics, and GA4 provides automatic event tracking and a connection to BigQuery.