These are instructions on how to jailbreak your iPhone 4 on the 4.2.1 firmware using RedSn0w for Mac. You will only be able to perform a tethered jailbreak at this time. This means you will need Redsn0w’s assistance to boot into your jailbroken device.
If you want to jailbreak a lower firmware version you can find the appropriate tutorial just keep scrolling down the page.
Make sure not to update to iOS 4.2.1 if you need an unlock as one is not yet available!
Create a folder on your desktop called Pwnage
Download RedSn0w from here and place it in the Pwnage folder. Likewise, download the latest 4.2.1 firmware from below and place it in the same folder.
4.2.1 (4): iPhone3,1_4.2.1_8C148_Restore.ipsw
Once iTunes has finished updating your iPhone to the desired firmware open the Pwnage folder on your desktop and launch the redsn0w application from the redsn0w folder we extracted earlier.
From this window you can select the jailbreak options you would like.
RedSn0w will now guide you through the steps to get into DFU mode. You can find more help with DFU mode here