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:
Step 3. For Windows, you will need to make sure to install the Nexus S 4G drivers. (See video for detailed howto.) The Nexus S 4G drivers are included in the zip file under usb_driver folder. For Mac and Linux, you don’t need to install drivers so skip this step.
Step 4. Open up a command prompt or terminal and browse to the directory where you unzipped the files.
For Linux and Mac:
Next, yype each of the following lines and hit Enter.
fastboot flash boot boot.img
fastboot flash system sytem.img
fastboot flash recovery recovery.img
Step 5. Also type the following and hit Enter after every line:
fastboot flash userdata userdata.img
fastboot erase cache
fastboot oem lock
Step 6. You should be fully unrooted, back to a stock ROM with your personal data erased.Need Help? Follow/add me on Google+, Facebook, or Twitter!
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You will need a rooted Nexus S to install all ROM/kernels.
NOTE: PLEASE ONLY INSTALL ROMS INTENDED FOR YOUR VERSION OF Nexus S! Otherwise you WILL BRICK YOUR PHONE and I will not be responsible!
How to Root Nexus S!
First time to rooting and custom ROMs? Please see our Nexus S FAQ FIRST! Nexus S ROMs
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Nexus S 4G ROMs