Kaatje, a former employee of Apple have jailbroken iPhone OS 4.0 Beta 4 on a iPhone 3GS! But it is still a work-in-progress thingy, which she have mentioned it is not prefect yet. Quite a few cydia apps can be installed but some might crash, thus she is not planning to release this version of jailbreaking tool. We ain’t too sure if she will be releasing a version of her jailbreaking tools, but lets wait and see if she release it.
Check out her blog for more information!