Makes me smile when reading the seems to be standard reply of don't buy a 4 or 4s because the 5/new iPhone is due out. Advice based on what ? speculation , rumour and Chinese whispers exist regarding the next iPhone.
We know not of what's under its hood or how it looks aesthetically or when it will definitely be released.
If one waited to see what's out new around the corner then one would actually never make a purchase at all.
In the Uk iPhones hold their price very well and thus selling your 3 gs/4/4s will generate a decent amount of cash to fund your next iPhone purchase, and allow someone further down the chain to own a great device.