It appears that a from Amsterdam by the name of Kaatje, has managed to create a custom firmware that allows iPhone 3GS users to jailbreak the newest iPhone OS 4.0 Beta 4.
The iPhone Dev team released Redsn0w 0.9.5 to jailbreak iPhone OS 4.0 Beta 4, however it was only compatible with the iPhone 3G. With Kaatje's work, we now have reports of a jailbroken 3GS running 4.0 Beta 4. And to top it off, she has managed to do so without upgrading the baseband, thus preserving the ability to unlock on blacksn0w if you wish to go back or stay on the safe side.
The method she describes requires a custom firmware using a patched PwnageTool. The patch will not work with the iPhone 3G, but thats what the current redsn0w release is for.
Although it's great news and fun for us users on 4.0, not all apps will be compatible of course. Cydia still has some issues loading and many applications install but fail to load and end up crashing, Those of you wanting to use apps such as mobileterminal, sbsettings, five icon dock will be out of luck for now.
Note: She will NOT be releasing this custom firmware at this point due to prior agreements with the dev teams and of course for legal reasons and because they gave her many parts of the jailbreak.