In GA4, user ID is a unique identifier assigned to a user that allows you to track their activities across devices and platforms. Unlike the GA3 tracking method, where cookies are used to track user data, GA4 uses a combination of cookies and user ID to track user data. When a user logs in to your website, you can assign them a User ID. This User ID is shared between the user’s devices and allows GA4 to track their activities across devices, even if the user clears their cookies. The User ID also enables you to build a more complete picture of your users’ behavior across multiple sessions and devices. This can help you understand how users engage with your website or app, identify trends and patterns in user behavior, and optimize your marketing and advertising efforts.