Create a DFU IPSW to use with iTunes. Go to Extras > Even More > DFU IPSW, select the iOS 4.2.1 IPSW. RedSn0w will create a ENTER_DFU_iPhone2,1_4.2.1.xxxxxx.IPSW. Open up iTunes, shift+click Restore and point it to this new DFU IPSW. Let the restore process go through. At the end of it, you'll be in DFU mode. Once you're in DFU mode, follow the guide, except you ignore the parts where it says to put the device into DFU mode since you're already in DFU mode.