A few days back, Sprint announced the update to Ice Cream Sandwich was ready to go, and today Jean-Baptiste Queru (the AOSP guru at Google) has announced that the sojus factory images are available as well, bringing the Nexus S 4G up-to-date with the GSM versions.
These aren't files to update your phone. On the contrary, they are there to restore your phone back to stock if you've knocked it out of whack whilst fooling with it. As JBQ recommends, if you're running a stock version of Gingerbread on your NS4G, just wait for the OTA -- it's coming. But if you're the adventurous type, you could use these images to get your NS4G to the exact state it would be if all OTA updates were applied -- ICS IMM76D. It takes a bit of keyboard wizardry while in Fastboot, and it will wipe everything. Know this going in.
No matter which way you're going to roll with it, you'll find all the help a person could ever want in the Nexus S 4G forums. High-tail it in there and join in the frenzy.