I've deployed my Symfony2 website to an external development server, however it hasn't pulled the app_dev.php file from my github repository. app.php has been pulled and I can access the site from there but if I go to /app_dev.php I get a 404 not found because the file does not exist on the server. The file is definitely in my github repository though and is not within my .gitignore file.
Capifony removes app_*.php files for security reasons. You shouldn't use them in production.
If you really need them, you can set the clear_controllers option to false:
set :clear_controllers, false