Hello ej86, welcome to the forums!
That data usage is not right and it looks like you might want to get a handle on it pretty quickly!
Here is an S4 forum, see what they can tell you! Samsung Galaxy S4 - Android Forums
I am not sure what methods your phone has of checking on your data usage. I do not know how you can tell which app is eating it. Probably something you can turn off though. I think you should be able to do something like Menu/settings/ Accounts and sync and see what all you have going on. Need to turn something off!
If you can get on to a wifi network, that does not use your plans data either.