Whichever you want to make available, 4 or 6 is still a small enough menu. The only comment I would make is to order the groups of 6 either by common separators, might make it easier to find what the user is looking for, eg:
``` yyyy-mm-dd dd.mm.yyyy mm.dd.yyyy dd/mm/yyyy mm/dd/yyyy yyyy/mm/dd ``` or order by date arrangement ``` yyyy-mm-dd yyyy/mm/dd dd.mm.yyyy dd/mm/yyyy mm.dd.yyyy mm/dd/yyyy ``` whichever seems to work for users.