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If you have Applecare +, it covers 2 accidental damage each at $49.00. After that the replacement cost will be $199.00 for the 4S.
 
If you have a 16GB iPhone 4 - does Apple replace it with a 16GB or the 8GB?

Your replacement will be the same as what you paid for. Common sense on this one. However, mistakes do happen, so look at the replacement carefully.
 
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