However when I direct my browser to mysite.com/ps.php, I get a 404 error. If I remove the mod rewrite part, however, it works fine. Can I add an exception to the mod rewrite part to make it ignore /ps.php.
ps.php should NOT be redirected if it exists OR it should be redirected to index.php (where WP can parse the URI to decide what to do with it). You should NOT be getting a 404 (based on what you’ve shown).
Okay, can you add an exception for ps.php? You already have as the RewriteCond %{REQUEST_FILENAME} !-f says to redirect if ps.php is NOT a file.