Data Model: GA4 follows an event-based data model, where data is organized around events and parameters associated with those events. UA, on the other hand, uses a hit-based data model, where data is organized around individual hits sent to the server.

User-Centric Tracking: GA4 adopts a user-centric approach, utilizing an anonymous user ID to track users across multiple devices and platforms. This allows for better cross-device and cross-platform attribution. UA primarily relies on cookies and client IDs for user tracking.

Measurement Capabilities: GA4 offers enhanced measurement capabilities compared to UA. It includes built-in event tracking for common interactions such as scrolls, video engagement, and file downloads, without requiring additional customization.

Data Streams: GA4 introduces the concept of data streams, which allows different types of data (e.g., app data, web data, offline data) to be sent to separate data streams within the same property. This provides more flexibility in data collection and management.

Reporting Interface: The reporting interface in GA4 differs from UA. GA4 features an updated and customizable interface, providing new insights, analysis tools, and visualization options. It focuses on exploring user behavior and engagement across the customer lifecycle.

These differences highlight the evolution of GA4 as a more advanced and user-centric analytics platform compared to UA.

GA4 is free. However, there are certain limits, and if you do hit those limits, then you might consider upgrading to the paid version of GA4.

You should switch to GA4 as soon as possible. Google will discontinue Universal Analytics on 1st July 2023 and there will be no backward compatibility for UA data. So an earlier switch means more historical data.