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);