The problem isn't the hardware, but the fact that Sprint SIM locks the 4S phones they sell. They will unlock on request, but ONLY for non-US carriers. Even with an "unlocked" Sprint 4S, there are still restrictions in place that prevent those models from working on Verizon, or AT&T, or any other US carriers.
The restriction is software only, not hardware. And yes, it's a Sprint decision.
Also: the fact that iPhone was never activated doesn't mean anything. It'll still have to be activated and that means going through Apple's servers. When Apple sees your 4S trying to activate, it will note that the MEID is allocated to Sprint and try to activate through them.
You could give Gevey SIM and similar options a try, though that's at your own risk.