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<?php

add_filter( 'gpui_unique_id_attributes', 'my_custom_unique_id_attributes', 10, 3 );
function my_custom_unique_id_attributes( $atts, $form_id, $field_id ) {

	// if this is our desired field
	if( $field_id == 1 ) {
		// bypass 'numeric' ID min character length of 9
		$atts['length'] = 4;
	}

	// return modified attributes
	return $atts;
}