After the recent iOS 6 beta 4 release, MuscleNerd, the famous iPhone hacker from L A, stated that redsn0w 0.9.13 dev 3 developers version still jailbreak iPhone 4 running iOS 6 beta 4, but must be used an iOS 6 beta 3 ipsw.
Unfortunately, this beta version will not install the Cydia application because Dev Team does not think developers need an automatic installation of the platform, but following this tutorial you can install Cydia yourself.
Those who have installed Cydia on iOS 6 beta 3 said that the application is not fully functional and don’t think the problem was solved for iOS 6 beta 4, so be prepared for the same result.
You can download redsn0w 0.9.13 dev 3 Windows / OS X from here.