I would like put some files within the folder wordpress/crontab/ have cron job. And So I need all of the files under wordpress/crontab/ can run well and never pointed right into a 404 page. This is actually the Rewrite rule in wordpress .htaccess, how you can then add line that permit folder wordpress/crontab/ out from the rule? Thanks.

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /wordpress/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /wordpress/index.php [L]
</IfModule>

# END WordPress

Okay, I'll publish this being an answer too:

What @kb stated (within the comments) is appropriate, appears like you ought to be being able to access localhost/crontab/text.php rather, due to the RewriteBase set to /wordpress/

Cheers.