This step by step guide/tutorial will show you how to update your iPhone 4 to iOS 4.1 while preserving the baseband. Because we will not update the baseband, you will still be able to run the latest iOS version available, you will be able to jailbreak using LimeRa1n, and you will be able to unlock your iPhone using UltraSn0w.
This method works for both Windows and Mac. It is pretty simple but it does require you to read the steps carefully. As usual when hacking your iPhone, you are proceeding at your own risks. This being said, this is how to upgrade to iOS 4.1 without upgrading the baseband...