iPhone-Dev-Team has just released the new version of Redsn0w(v 0.9.5b4) which will jailbreak the latest iPhone OS 4.0 beta 4. Last week Apple released iPhone OS 4.0 beta 4 fixing the loophole in beta 2 & 3 which allowed the hackers to jailbreak them via Redsn0w 0.9.5b1 and Redsn0w 0.9.5b2 respectively. Redsn0w 0.9.5b4 will jailbreak only iPhone 3G running iPhone OS 4.0 beta 4.
Please note that redsn0w 0.9.5b4 is for jailbreaking iPhone 4.0 Beta-4 OS only, it's not meant for unlocking your iPhone running OS 4 beta 4, so if you want to use your device in unlocked state, please refrain from upgrading to OS 4.0. Procedure to jailbreak iPhone 3G on OS 4.0 Beta 4 is similar to the step-by-step procedure to jailbreak iPhone OS 4.0 beta 1 with redsn0w 0.9.5 given here.
To find more about iPhone OS 4.0, please visit here.
Download redsn0w 0.9.5b4 for Mac
Download iPhone OS 4 Beta 4.
Update 1: Jailbreak iPhone 3G iOS 4.0 GM with Redsn0w 0.9.5 on Mac
Update 2: Jailbreak iOS 4.0 with Redsn0w 0.9.5b5-2 on Mac | Windows
Update 3: Jailbreak iOS 4.0 with Redsn0w 0.9.5b5-3 [Hacktivated]
Update 4: Redsn0w 0.9.5b5-4: Fix iBooks Issues After iOS 4.0 Jailbreak
Update 5: Download Redsn0w 0.9.5b5-5 to Fix iOS 4 APN / MMS Issues