Yes, the 4S is dual CDMA/GSM. In the US, it will only ever work in CDMA mode. Overseas it will roam onto GSM networks, though it will prefer a foreign CDMA network over a GSM network if one is available (you can turn this off in Settings).

It's by default locked to only take Verizon SIMs. You can call Verizon and they will unlock it if you've been a customer in good standing for at least 60 days. Call Verizon Global Services at 800-711-8300 and ask for a SIM Unlock. If the rep is confused, explain that you want to use local, foreign SIMs in your phone when traveling outside the USA.

In order to apply the unlock, you will have to put in a foreign SIM when your phone has access to a wifi network. ONce the SIM is in, your phone will lead you through the activation process and unlock itself.

Note that the Verizon SIM unlock will only work for SIMs not based in the USA. You can use a T-Mobile Germany SIM but not a T-Mobile USA SIM, for example.