Google Analytics 4 is Google's fourth-generation analytics platform, mandatory since Universal Analytics stopped processing data on July 1, 2023. Unlike UA's session-based model, GA4 uses an event-based architecture where every interaction—page views, clicks, scrolls, video plays—is an event with parameters. This shift makes cross-platform tracking between websites and mobile apps seamless under one property. GA4 was built for a privacy-first world. It doesn't rely on cookies the way UA did, uses consent mode to model data gaps when users opt out, and anonymizes IP addresses by default to align with GDPR and Canadian privacy laws. Google also baked in machine learning to predict user behavior, like purchase probability or churn risk, even with incomplete data. The interface confused a lot of people at first. Standard reports are limited compared to UA—you'll spend more time in Explorations (custom reports) to dig into real insights. Conversions work differently too: you manually mark events as conversions instead of setting up goals, and GA4 uses a data-driven attribution model by default rather than last-click. Key differences practitioners notice: - Events replace hits and pageviews as the core data structure - BigQuery integration is free (huge for agencies running multi-client analysis) - Session definition changed—30-minute timeout stays, but campaign changes no longer start new sessions - Bounce rate is gone; "engagement rate" (sessions over 10 seconds, with conversion, or 2+ page views) replaced it At Ottawa SEO, we migrated all 500+ properties before the UA cutoff. The learning curve was real, especially rewriting conversion tracking and rebuilding dashboards, but GA4's predictive metrics and better app/web unification are worth it once you're past setup. If you're still on UA historical data, export it—Google won't keep it forever. GA4 isn't optional anymore; it's the only game in town for Google's ecosystem.