Actually, if we're speaking about AT&T, RIM doesn't need a new radio, it won't have any affect whatsoever on battery life, and can be accomplished via a simple OS upgrade to display "4G" on the homescreen instead of 3G, anytime they want to do it. I'll actually go out on a limb and say you'll probably see it rebranded that way soon - as that's exactly what AT&T has done here. Its all marketing-speak. AT&T has quietly been rebranding their existing 3G network as 4G without any new technology deployments, and are coming out with rebranded phones too.
So, your AT&T 9900 is/will be "4G speed" whether you knew it or not.
AT&T Pins 4G Label to Existing Network