It looks like the wait for Ice Cream Sandwich on the Nexus S 4G is almost over: What seems to be an official Android 4.0 ROM has leaked onto the xda-developers forum. The ROM reports its OS version as 4.0.4, and also comes with an update to the handset's baseband. Google started pushing out an Ice Cream Sandwich update to the GSM version of the Nexus S back in December, and although ICS has been available via third-party ROMs for a while now, there's been no firm date set for an official update for the 4G.
Initial reaction from the xda-developers forum has been positive, with no complaints of instability, and reports of increased WiMAX speeds with the new baseband installed. If the performance of the leaked ROM is anything to go by, Sprint can't be too far from pushing this update out to the public. If you can't wait for the official OTA update, and you're aware of the risks involved in flashing your phone, you can head over to the source link to try the ROM out for yourself.