I believe the way to create a custom 404 error page for a specific directory would be to configure that within a Directory directive. That can be also inside the VirtualHost directive as well.
It might be something like the following:
<VirtualHost 192.168.5.5:80>
ErrorDocument 404 /404a.php
<Directory /path/to/directory>
ErrorDocument 404 /404b.php
</Directory>
</VirtualHost>
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