functions.php

/**
 * Add Google Analytics GA4 Tracking Code to Header
 */
add_action( 'wp_head', 'add_google_analytics', 10 );
function add_google_analytics() {
    ?>
    <script async src="https://www.googletagmanager.com/gtag/js?id=G-XXXXXXXX"></script>
    <script>
      window.dataLayer = window.dataLayer || [];
      function gtag(){dataLayer.push(arguments);}
      gtag('js', new Date());

      gtag('config', 'G-XXXXXXXX');
    </script>
    <?php
}