They have access to 5.5 gbs, not 4.5.
They also have the ability to decrease the RAM footprint over time. The only reason they're reserving the extra 1 gb for the OS is to have the option of adding features should some kind of killer new feature become popular. Once you allocate more RAM to games, you can't take it back for the OS. There's also rumor that they could clock the GPU higher with an update. Not really THAT big of a deal tbh. The bandwidth advantage and GPU advantage is huge compared to the Bone.
EDIT: Unrelated, but I guess MS really liked Sony's "The best place to play" slogan for PS4. So they just copied it outright.
More like "the best place to be spied on by the NSA"