/**
 * Hide flat rate rate when advanced free shipping is available
 */
add_filter( 'woocommerce_package_rates', 'mx_unset_flat_shipping_when_free_is_available', 10, 2 );

function mx_unset_flat_shipping_when_free_is_available( $rates, $package ) {

    if ( isset( $rates['advanced_free_shipping'] ) ) {
    unset( $rates['flat_rate:4'] );
}

return $rates;

}