Will the CDMA Verizon iPhone 4 work on the Sprint network? Yes. Will you be able to use it on the Sprint network? Probably not.
The CDMA iPhone (Verizon), unlike the GSM iPhone (AT&T, Orange, etc), isn't locked. That said, CDMA providers have full control over what devices operate on their network and don't typically allow outsiders.
So while you CAN use the Verizon iPhone 4 on other CDMA networks, you probably won't be able to.