To retrieve the GA4 client_id using Google Tag Manager (GTM), follow these steps:

1. Create a variable in GTM of type 1st Party Cookie with the Cookie name set to _ga.

2. Create another variable of type Custom JavaScript with the following code snippet:

   function() {
       return {{COOKIE _ga}}.substring(6);
   }
   

After setting up these variables, the client_id will be extracted from the _ga cookie and can be used in other tags and platforms, respecting the user's cookie consent.

The GA4 client_id format is a unique identifier that looks like this: GA1.2.1839138233.1715355754. The actual client_id starts from the sixth character, after GA1.2. It is stored in a cookie named _ga and helps GA4 to recognize if the same user has visited the website over time.

The GA4 client_id serves as a unique identifier that is used by Google Analytics 4 (GA4) to determine whether the same user has visited a website over time. It is saved in a cookie called _ga and is crucial for tracking individual user behavior across sessions.