Along with the recent iPad 3New iPad announcement, iOS 5.1 was also released.
As the picture above suggests, updating your iPhone 4s to the latest software will allow you to access 4G data speed, impossible right? It's not like the update adds an entirely new piece of hardware?
Unlike The New iPad, the iPhone cannot connect to LTE 4G so, Apple has begun to brand HSPA+ network connectivity (found on AT&T) as 4G, in a fearless bid to capture the 4G phone market over in the US: currently Apple doesn't have a 4G phone to compete with the many Android 4G enabled devices. In fact nothing has changed apart from the icon, the iPhone 4s has always been able to connect to these networks.
While fast (14.4Mbps), HSPA+ isn't technically 4G, it's more 3.5G. Real 4G, LTE, has been proven to reach speeds of 30Mbps in real world speed tests, so there's not really too much competition. Of course, unless you're downloading massive files to your phone you probably won't even notice the difference.
So to get 4G a flashy 4G icon on your phone, update the OS and you're set! (In US only, of course)