My iPhone 4S using 4G after software update

I just helped another customer of upgrading his iPhone 4S to the latest software update of iOS. After the software update, still in 3G network, and he's with Sprint carrier. I thought Sprint are already in 4G for iPhone 4S?

At this time, only AT&T upgraded iPhone from 3G to 4G network. The Verizon and Sprint stay with 3G network after the iOS software update to 5.1

Just in case you missed reading article on how to update to version 5.1, here's the link: http://www.whaddya.com/2012/03/iphone-ipad-ipod-software-update.html