Unlock a Virgin Mobile USA iPhone 4s -- worthwhile?
iPhone 4s, both the CDMA and GSM models, has GSM hardware built-in. The only difference between the two models is whether the device supports CDMA, and in that case, which network it supports. A device sold by Virgin Mobile, which piggybacks on Sprint's network, may work in CDMA mode on the Sprint network once unlocked, though you would need to confirm this with Sprint. It cannot be used on Verizon's network. In GSM mode, it can be used outside the US on essentially any network. It cannot be used on US-based GSM carriers (T-Mobile/AT&amp;T).

The iPhone 4 CDMA model (sold at Verizon) was not compatible with any GSM network, US-based or otherwise. That may be where the confusion lies.