Going with its matching OTA, the factory image of 4.0.4 for Nexus S 4G is available at the usual location
https://code.google.com/android/nexus/images.html
This is meant to be used by people who built and flashed their own custom version of AOSP and wish to return to the factory state. If you're currently running an official retail build, I simply recommend that you wait for the OTA.
Because this isn't actually my job, I don't know the exact schedule for this or any other OTA. Thanks for not asking about those.
Specifically about Nexus S 4G, there's a known bug in the bootloader, where it reports the wrong radio version number after flashing the radio, which then doesn't get recognized by fastboot. The workaround is to boot into whatever version of Android is on your device, and then return to fastboot and restart the flashing sequence.