How to switch at&t carrier on iphone 4 to sprint carrier?

I have an iphone4 with at&t carrier and would like to connect it with Sprint if possible.
My phone is jail broken and I unlocked it too but it still says at&t carrier on it and sprint won't connect it if its am at&t phone.

Sprint has terrible reception, be sure that's what you want to do before your bound by contract.

Not possible.
att is gsm and sprint is cdma… Its impossible.
sell it and buy a sprint iphone

No it is not possible cos sprint is cdma network.