Today a new version of Redsn0w has been released, Redsn0w 0.9.5b4. This new version of Redsn0w is created for developers that use iPhone OS 4.0 Beta 4. Beta 5 is probably coming very soon, so if you would like to try this jailbreak for Beta 4 before Beta 5 is released, you better be quick. You can download Redsn0w for Beta 4 here.