I checked Samsung's site... If I read it correctly, the sku for the developer edition is something like SCH-I545ZKDVZW. So I doubt that a working display phone would be the same as a developer phone, it would make no sense for them to do that. IMHO it would be a bit excessive to use a developer phone just show what the phone can do (for the most part).
So how much of the display phone actually is functional? Can you actually use it on the network at all? Is there anything that isn't working at all?
Lastly is it legal for a display phone to be sold to end users? I mean it doesn't seem right in some sense that a working display phone for the still being sold Galaxy S4 being sold to end users? Maybe it is legal or legit to do but I do hope you didn't get screwed over by this sale.
Galaxy S4 - Verizon
Rom - HyperDrive RLS9.1
*Google Wallet Mod - Xposed Addon
*sgs4-lib_fix-w1.6-R96-v13 (for Google Wallet Support)