The just-released iOS 5 beta 4 can be Jailbroken using the redsn0w tools which were updated for the new release. The guide and download links are below. iOS 5 is in beta and should only be used by developers to test apps in the new iOS.
It should also be noted that if you jailbreak your device on iOS 5, you wont get OTA updates from Apple, as explained by MuscleNerd of iPhone Dev-Team:
The 5.0b4 OTA update won’t work on JB devices due to an explicit check of /Applications..almost like a present from Apple
NOTE: It appears that by design, the OTA update that became available starting with iOS 5 beta4 will *not* be automatically applied to jailbroken devices. That’s a relief to those who don’t want to lose their jailbreak via OTA pushes. If you’re jailbroken, you’ll need to use the standard iTunes method to get to iOS 5 beta4.