define "Cell broadcast". Are you asking how to turn on or activate the cell feature of the iPhone 4S? Are you asking how to enable "Cellular data"? If the latter, go to Settings - Cellular and then enable the toggle button next to "Cellular Data". If the former, you need to notify the cellular provider for whom the iPhone is locked to or the one you'd like to have cellular service with. If neither, please explain in greater detail what you're asking.