For those of you who want to return your Nexus S 4G to completely stock state, you can by flashing stock files in fastboot.
This method works on Windows, Mac, or Linux.
*Warning – This unroot method is ONLY for Nexus S 4G and will erase all your personal data/apps/settings.
Step 1. Put your Nexus S 4G into fastboot mode by turning off your phone then rebooting into bootloader by holding down Volume Up and Power button together.
Step 2. Download nexuss4gunroot.zip and unzip it to the root folder of your hard disk: