There are major differences between the iPhone 3 and 4 models, beginning with the first noticeable one that is the style. The iPhone 4 have a stainless steel body wrapping it with more of a square bit of styling whereas the iPhone 3 was more rounded. It also means the iPhone 3G cases and other iPhone covers are not interchangeable. Once you get beyond the looks of the two, the internal differences are huge as well, which is nothing new with newer technology pieces as the technology is constantly changing and updating. The iPhone 4 has better graphics, works faster with higher-powered processors, and has more memory.
When purchasing iPhone covers make sure you know exactly which model you are buying for, not that it is a problem for most people, but this could be a problem for someone who is buying the iPhone covers as a gift for someone else. Since the iPhone 3G cases have to cover the more rounded body of the iPhone, they will not work on the flat, squarer body of the iPhone 4. In many cases, the iPhone covers being used for the iPhone 4 models are more of a skin style that is made out of vinyl or some other form of plastic.