Depends on your usage. For web, email, music, and movies only, the 4 is plenty fast and powerful. The screen is great. It's considerably heavier and bigger however. But if you are planning to use it mostly at home and don't require maximum portability, it's a fine machine, especially at $300, which if brand new, is quite a bargain.
But if you do get the 4, whatever you do, don't go into an Apple store and play with the Air. 😀