A hacker in Russia was able to get iOS 6 3D Maps running on an iPhone 4. He hasn’t managed to get the turn-by-turn navigation working, but it’s not as if we’ve had that in iOS before anyway.
There are instructions posted on how to do it on your iPhone, but it doesn’t sound exactly easy. You will also need iOS 6, which requires a developer account, and the iOS 6 jailbreak. All of that will require some work. At the very least, you may find yourself forking over $100 for an incomplete version of iOS 6.