The iPhone 6 will technically work on all 4 carriers if its unlocked. It has the hardware for both CDMA and GSM carriers
However, Verizon and Sprint will need to activate it on their CDMA portion. AT&T and T-Mobile would just need to issue a nano sim card with the service attached to it.
I believe Verizon will activate it without much hassle, but I don't know what Sprint's BYOD policy is currently for CDMA activation. It appears iPhone 6 can be activated with them though.
https://www.sprint.com/landings/bring-your-own-phone/
https://www.verizonwireless.com/bring-your-own-device/
