The iPhone Dev Team is back at work, again. Just like iOS 5 beta 3, the latest beta 4 build can be jailbroken by redsn0w, as noted by @musclenerd. This is a tethered jailbreak, and should only be used if you require a tethered jailbreak for iOS 5.
As described on the iPhone Dev Team blog:
Update #4: In conjunction with iOS5 beta4 being released to iOS developers, redsn0w 0.9.8b4 is now available for jailbreak app developers (point the b4 redsn0w at the b4 IPSW). Remember, itâ€™s a tethered jailbreak right now so youâ€™ll need to use redsn0w to boot into a jailbroken state at each power cycle.
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. Thatâ€™s a relief to those who donâ€™t want to lose their jailbreak via OTA pushes. If youâ€™re jailbroken, youâ€™ll need to use the standard iTunes method to get to iOS 5 beta4.