Add the following code to functions.php or to a custom plugin to style WooCommerce checkout form fields with sweet Bootstrap 4 styling.
/**
* Add Bootstrap form styling to WooCommerce fields
*
* @since 1.0
* @refer http://bit.ly/2zWFMiq
*/
function iap_wc_bootstrap_form_field_args ($args, $key, $value) {
$args['input_class'][] = 'form-control';
return $args;
}
add_filter('woocommerce_form_field_args','iap_wc_bootstrap_form_field_args', 10, 3);