4,5 GB is the whole game, not only the patch.
Steam has this issue, that if you didn't have it before and install it directly together with Civ, it will try to download Civ instead of installing it from the DVD.
In most cases, if you now cancel the download and try it again, it should install from the DVD.
If it doesn't, then remove Civ5 in the steam library, uninstall steam, download it, install it again, and install afterwards Civ5 from the DVD.
Yeah, great design, isn't it?