UPDATE: Apple released iOS 6.1.3, so you can NOT restore to earlier firmware (both original and custom), unless you have saved your SHSH keys. If you are on iOS 6.0-6.1.2 you can use evasi0n to jailbreak.
Using this tutorial you will be able to create a custom firmware, untether jailbreak and unlock iOS 6.1.2 running on iPhone 4. This tutorial uses Sn0wBreeze 2.9.11 for Windows.
Sn0wBreeze 2.9.11 allows users to preserve modem version for unlock.