Cheapest i've seen this and have been looking for a while. Just remember that this is not going to give you absolute stonking sound from your iPhone but for £6.64 what can you expect.
And no, it's not compatible with the iPhone4
Edit: This is sold on Amazon by Prepaymania at £6.64 inc del. However on the prepaymania website, they are selling it for £7.99. So a bargain!