Very easy, as long as the USB data cable connection is not via any USB hub. But directly connected to Computer USB port.
Get RedSn0w. Get 4.2.1 IPSW file. Use RedSn0w's Extras section to Load the IPSW file, and then go back previous screen to Jailbreak. There are additional features that are cool to choose: boot logo (png file), multitask, homescreen wallpaper, etc. Follow the rest of the instructions on RedSn0w.