iPhone Dev-Team has just Released the New Beta of the Redsn0w 0.9.5 to Jailbreak iPhone OS 4.0 Beta 4 and it supports iPhone 3G, Mac OSX, the iPhone OS 4.0 beta1-beta4 FW. But there is no Windows version of this beta redsn0w (since apps are developed on MacOSX) This beta release of redsn0w targeted at developers of jailbroken apps.
Here’s some words from the iPhone Dev Team
This is a beta release of redsn0w targeted at developers of jailbroken apps. It uses the same pwnage2 DFU-mode exploit that we’ve been using since the 2.x days. Nothing new is revealed to Apple, and it’s not applicable to the iPad. Don’t go near it if you use yellowsn0w, ultrasn0w, or blacksn0w.
YOU SHOULD STAY CLEAR OF THIS BETA SOFTWARE IF YOU RELY ON A CARRIER UNLOCK. That’s because the beta redsn0w works only if your device has already been upgraded to the stock 4.0beta1/beta4 IPSW from Apple, which contains a baseband update. If you installed that IPSW, you’ve already lost the carrier unlock until the next planned release of ultrasn0w and blacksn0w.
Download Redsn0w 0.9.5b4 for Mac [Fast Mirror link]
Download iPhone OS 4.0 Beta 4