How to hide certain file type using apache .htaccess?

<FilesMatch "\.(htaccess|htpasswd|ini|log|sh|inc|bak)$">
Order Allow,Deny
Deny from all
</FilesMatch>

Loaded with some common extensions, add more as you need.

If you really want a 404 (and not a 403), you can use mod_rewrite:

RewriteEngine on
RewriteRule \.ext$ - [R=404]

<FilesMatch "\.ext$">
    Deny from all
</FilesMatch>

See the Apache documentation on FilesMatch and Deny

EDIT: Artefacto makes a good point, this will return 403, not 404, if that's important for your purposes

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