For the iPhone 6 trade-in at Verizon, does it have to be a Verizon iPhone 4S or does it not matter?

I currently have an iPhone 4S (Sprint) that I'd want to trade in for the new 6, but will Sprint take my phone back? And will Verizon accept a Sprint version?
Also, if the screen is cracked, but fully operational, will they accept it?

Sorry for having so many questions…

Dunno