Hi,

You should paste this code into the function.php of the child theme or via the CodeSnippet plugin here: https://wordpress.org/plugins/code-snippets/

/**
 * Combine category posts with posts tagged with tag ID 4
 * Works on category archive pages only.
 */
add_action( 'pre_get_posts', function( $query ) {
    // Only modify the main frontend category archive query
    if ( 
        ! is_admin() 
        && $query->is_main_query() 
        && $query->is_category()
    ) {
        // Get current category ID
        $category = get_queried_object();
        $category_id = $category->term_id ?? 0;

        // Combine category + tag=4
        $tax_query = [
            'relation' => 'OR',
            [
                'taxonomy' => 'category',
                'field'    => 'term_id',
                'terms'    => $category_id,
            ],
            [
                'taxonomy' => 'post_tag',
                'field'    => 'term_id',
                'terms'    => [4],
            ],
        ];

        // Set the new tax query
        $query->set( 'tax_query', $tax_query );

        // Ensure posts are ordered by date (newest first)
        $query->set( 'orderby', 'date' );
        $query->set( 'order', 'DESC' );

        // Optional: define posts per page if needed
        // $query->set( 'posts_per_page', 10 );
    }
});

/**
 * Remove duplicate posts if they appear in both category & tag.
 */
add_filter( 'posts_distinct', function( $distinct, $query ) {
    if ( 
        $query->is_main_query() 
        && $query->is_category() 
    ) {
        return 'DISTINCT';
    }
    return $distinct;
}, 10, 2 );

Replace [4] with the tag ID that you want to show.

Regards,
PenciDesign.