What is the correct way to create a translatable in four languages website logo with slogan?
Are you already using the i18n module? The package contains the 'Variable translation' module which allows you to make certain variables language dependent. Since the site logo is also stored in the variables (theme_settings['logo_path'] if I'm not mistaken) you can make that variable language dependent as well.

If you're not using the i18n module (and you're not going to need it soon), I suppose you can also switch between logo images in your theme's template.php.

I wouldn't recommend creating a node type for it. In my experience, whenever you're considering to create a node type that is meant to have a specific number of nodes, there probably is a better solution for what you're doing.