I just downloaded the lastest trial of the SL4 version of Telerik controls. The assembly versions are 2010.1.0422.1040.
I'm using the VS2010 Release(10.0.30319.1). I created a new SL4 project, ran the utility to make Telerik controls available, and I am able to use the controls just fine, but RadRichTextBox is nowhere to be found. Is it not available in this release?
EDIT: Nevermind, I'm an idiot - I just had to manually add it to the toolbox. It's curious that all the other controls seemed to be automatically added the toolbox, but not that one.