How to create a postgresql function that increment a string lead with 0000

increment-string.sql

CREATE OR REPLACE FUNCTION public.after_update()
 RETURNS trigger
 LANGUAGE plpgsql
AS $function$
declare 
	maximum character(5);
	nextMax character(5);
BEGIN
   -- Find max urutan
	select max(tnp.urutan) into maximum from something;
	-- Increment max
	nextMax := lpad((cast(maximum as integer) + 1)::character(5), 5, '0');

	-- Handle 99999 value
	if (maximum = '99999') 
		then return null;
	else
 		insert into something;
		return null;
	end if;
END;
$function$
;