I want to redirect only one specific URl (ex. mypage.com/1234/page/) to error page 404. How to achieve it in htaccess?
With mod_alias activated, you can do that with the Redirect directive:
Redirect 404 /1234/page/
Redirect expects usually the target as 3rd argument, leaving it brings up the configured ErrorDocument. If a target is added it would result in an internal error.
As can be read in the docs:
If the status is between 300 and 399, the URL argument must be present, otherwise it must be omitted.
(Thanks @w3d for that note!)