Does the iphone 4s only use 4g or 3g to?

As far as I know, the phone uses the best available network. That's why you don't have the option anymore to switch off 3G network.

If iPhone is connected to the Internet via the cellular data network, the (UMTS/EV-DO ), (EDGE), or (GPRS/1xRTT) icon appears in the status bar.

The 3G (UMTS) cellular network supports simultaneous voice and data communications on GSM networks. For all other cellular connections, you can’t use Internet services while you’re talking on the phone unless iPhone also has a Wi-Fi connection to the Internet. Depending on your network connection, you may not be able to receive calls while iPhone transfers data over the cellular network—when downloading a webpage, for example.

copied from page 148 iphone_user_guide 5

The iPhone 4S can be considered a "4G" phone since it can produce "4G" LIKE speeds of 14.1 Mbps download speeds. It can do this on HSDPA or also known as HSPA+ which is the highest form of 3G speeds possible and touches into territory of "4G" speeds. True 4G however is either WiDi or LTE networks but the iPhone DOES NOT support WiDi or LTE. HSPDA is only available on GSM networks and is not available from CDMA networks such as Sprint or Verizon in the US. The only GSM network that can support HSPDA in the US is AT&T. So if you have the iPhone 4S in the US then on AT&T you can run at 14.1 Mbps but on Sprint and Verizon the iPhone 4S is still only able to run at 7.2 Mbps like the iPhone 4. So to answer your question the iPhone 4S is NOT "4G" so it does NOT switch back and forth between 4G and 3G, and it can only reach "4G" LIKE speeds on AT&T's super high speed HSDPA 3G network.