Why isn't device storage an option on my Galaxy S4?
I wonder that "device storage" turns up as an option at all, as it is not intended for direct user interaction. On internal storage, each app gets its "home directory" to write in, where they usually store their data and settings – but not e.g. your photos and documents you might wish to also access with other apps (as basically, on "device storage", each app is limited to its own directory).

Maybe you confuse this with "internal SDCard", which is the user-storage on the device (as opposed to the "external SDCard")? To get some idea on the terms, be invited to our tag-wikis on the terms storage, internal-storage, and file-system.