When 3D Maps was first announced in iOS 6, Apple stated that it would only be compatible with the iPhone 4S, iPad 2, and the new iPad. However, a Russian developer, Anton Titkov, has found a way to get 3D Maps working on the iPhone 4.
Titkov has created what he calls the 3Denabler, which allows iOS 6 beta users with the iPhone 4 to run 3D Maps. However, even with this it still lacks the turn-by-turn functionality. It’s not available through Cydia since an iOS 6 jailbreak doesn’t really exist yet and getting 3Denabler to run isn’t as straightforward as a traditional jailbreak. Also, the instructional videos are in Russian.
It’s important to note that this is by no means a sure thing as iOS 6 has not yet been officially released and Titkov’s 3Denabler is all based on beta software. Apple will likely block things like this in future beta releases.