Here is a link to the releases of the RadControls
Here is a link about Sitefinity's version
http://www.sitefinity.com/devnet/forums/sitefinity-4-x/general-discussions.aspx (Sitefinity 4.0 forum)
The controls written with Sitefinity's 3.x API cannot be plugged directly in Sitefinity 4.0. The API of Sitefinity 4.0 is different. The RadControls that we use are just a newer version.
the Telerik team