This step-by-step tutorial will show you how to jailbreak your iPhone 4 on Mac OS X in less than 10 minutes. Just follow the steps below:
1.) Download the following software to your “~/Downloads” directory:
2.) Click your cursor on the Desktop to bring up the “Finder” menu, and go to “Go” -> “Utilities” -> “Terminal“.
3.) Type the following commands into “Terminal“: mkdir ~/Desktop/redsn0w;mv ~/Downloads/redsn0w.zip ~/Desktop/redsn0w/;mv ~/Downloads/iPhone3,1_4.2.1_8C148_Restore.ipsw ~/Desktop/redsn0w/;cd ~/Desktop/redsn0w;unzip redsn0w.zip
4.) Now using “Finder“, open up the ~/Desktop/redsn0w directory and double-click on the redsn0w.app application.
5.) Once you have opened the redsn0w.app application, you should now see the redsn0w 0.9.6b4! application window. From here you will need to click on Browse and select the file ~/Desktop/redsn0w/iPhone3,1_4.2.1_8C148_Restore.ipsw and hit next.
7.) Now you will need to select “Install Cydia” and hit next and follow the on-screen instructions. Also once this process has finished you will need to perform steps 5 through 6 again and when you get to step 7, select “Just boot tethered right now“. Again, you will need to follow the on-screen instructions and this time your iPhone 4 will be booted up in tethered mode and you will have access to cydia and your iPhone 4 will have been successfully jailbroken.
###VERY IMPORTANT### Every time you need to reboot or boot up your iPhone 4, you will need to peform step 7 and use redsn0w.app to “Just boot tethered mode“. Your iPhone 4 will be able to go to sleep or be re-sprung without having to be booted up using redsn0w.app, but every time you need to perform a hard reboot of the device, you will need to use redsn0w.app to boot it up.###VERY IMPORTANT###
###DISCLAIMER: I have tested this procedure with iPhone 4 running iOS 4.2.1 only. I have done this several times and I have never run into any problems and I have never bricked an iPhone. However, if you use this tutorial, you are using it at your own risk. ###DISCLAIMER###