RedSn0w has been updated to version 0.9.5b4 by the Dev Team for the recent iPhone OS 4 beta 4. This beta release of RedSn0w is not for casual iPhone users. It is aimed at developers of jailbreak apps so they can update their applications to work on OS 4.
RedSn0w 0.9.5b4 jailbreak for iPhone OS 4 beta 4 uses the same pwnage2 DFU-mode exploit that has been using since OS 2.X, so nothing has been revealed to Apple.
Because this version is for iPhone OS 4 beta 1-4 which contain a baseband update, anyone who’s remotely interested in unlocking should stay away from the OS 4 betas or even this version of RedSn0w.
Further information and instructions can be found here.