Companies usually deliver a product of the same value or higher in case yours fails and you have to make use of the guarantee. In order to be given one of superior value, the requirement of not existing stock of your product must be met.
This is exactly what is happening in the United States and Canada where there are no more units of the iPhone 4 white is out of stock. If a user goes to an Apple Store to replace their iPhone 4 with a hardware failure, they will leave the store with a brand new iPhone 4S refurbished at no additional cost.
It is not known if this position will be transferred to the rest of Stores in which the stock of the white iPhone 4 begins to be scarce but it would be the logical thing to do.
Of course, the white iPhone 4 has to have some hardware failure to be replaced and the manipulation of the terminal entails an invalidation of the warranty. Be careful with doing the picaresque to get a more modern terminal for free.
Source: 9to5Mac