The short version is "no". You have to go to a Sprint store or call customer service and activate the GNex on your account and remove the iPhone 4S.
The long version is... On a GSM carrier (in the US that's AT&T and T-Mo) you can just switch your SIM card and use any phone. Sprint (and Verizon) are CDMA carriers which means things aren't that simple. There is no SIM-card for voice and 3G data, the phone itself has to be activated and put on the account by Sprint. There is a SIM card to allow you to access LTE data, but you still can't just swap them. I don't know about the iPhone 4S, but the GNex LTE-sim is internal and can't be accessed.