From ModMyi - Wiki

Fix the endless reboot

Download this file: http://www.nullriver.com/~adam/endless_reboot.zip and unzip it. It will expand to a folder called “endless_reboot.” Connect your iPhone to your Mac. Put your iPhone in recovery mode by holding down the sleep and home buttons simultaneously for about 25 seconds until the yellow triangle and “Please connect to iTunes” message appear. Launch the Terminal (located in /Applications/Utilities) In the Terminal, type cd, then a space, then drag the folder “endless_reboot” onto the Terminal window (you should see something like cd /Users/username/Desktop/endless_reboot/) and press return. Type ./iphonecomm -s 4 and press return