Your iPhone 4/4S will not break (even after 1,000 drops) unless the impact is on the front or back gorilla glass. Usually the front is the culprit since we all use cases.
If you seen the "will it blend?" iPhone 4 version, the iPhone just bounces around on the blender like it's nothing until the blade hits the front, and then it shatters into a million pieces.
I've dropped my phone a bunch and it's never broke, but it has never had it's impact on the face, either. That would be kind of random and hard to do.