Redsn0w 0.9.8b4 is required to jailbreak iOS 5 beta 4 which is released to developers recently.
Remember that jailbreaking iOS 5 will be a tethered jailbreak only.
Supported Devices for iOS 5 jailbreak:
- iPhone 4 (GSM & CDMA)
- iPod Touch 3G and 4G
- iPad 1G
Redsn0w 0.9.8b4 for Windows
Redsn0w 0.9.8b4 for Mac