You can't toggle this setting directly...perhaps if you're rooted you can, but not with the current release of ICS. You can, however, quickly get to settings screen where you can switch between modes. I'm using
LTE Switch (free in the market). It takes you to the 'Mobile network settings' screen where you can tap on 'Network Mode' and switch between LTE/CDMA (aka 4G mode) and CDMA (aka 3G mode).