How to reach 404 page through multiple htaccess rewrites - php

Question

I'm pretty novice at htaccess but I know enough to make a few simple redirects and rewrites happen. My problem is that I've never combined multiple rewrites, such as in the custom PHP/MySQL gallery system I've created, and so I'm having a few issues.
The main things I want to accomplish are to have the gallery system structured as such:
https://example.com/album
https://example.com/gallery
https://example.com/gallery/img/img-name
This is actually working successfully already, but it seems to also be catching any invalid URLs and assuming they're gallery names, so my 404 page goes completely overlooked. So for example,
https://example.com/asdf
brings up a blank gallery page when it should obviously produce the 404. Do I need to somehow have my htaccess file check my database for potential existing name conflicts? Is that possible? Or is there a simpler solution that I'm overlooking?
Here's what I have now:
RewriteEngine On
RewriteCond %{REQUEST_FILENAME}\.php -f
RewriteRule ^(.*)$ $1.php [NC,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/\.]+)/?$ /album.php?a_id=$1 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/\.]+)/?$ /galleries.php?g_id=$1 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([a-z-]+)/img/([a-z0-9-]+)/?$ img.php?g_id=$1&i_id=$2 [NC]
Options -Indexes
ErrorDocument 404 /404.php
Thank you in advance!