I've seen numbers about the full install size of the framework, but my Google-fu has failed me when it comes to how large the upgrade download will be. For instance, for a user who has 2.0 installed, how much will they have to download to upgrade to 4.0CP?
41 MB as shown on the download page.
Update: It has to download all 41MB no matter what. 4.0 is not an upgrade to 2.0 like 3.0 and 3.5 were. It's a standalone version.