Welcome to Android Central! You can use the SD card to save media files and individual documents. You might be able to move a portion of an app to the SD card, but this depends on the phone and the app. Go to the App Manager, select an app, and see if there's a Move to SD button. Remember, this only moves a portion of the app, not the whole thing.
SD cards can't be used to install an entire app on an S4 (unless perhaps you root the phone and use an app like Link2SD or App Mgr III).