I have now added the older version to your Silverlight Trial, and when you go to the trial "Download" link, you will see a "Go to all products versions page" link, from where you will be able to download the version that you need.
If you purchase a license for our RadControls for Silverlight, we will be able to add the older version to your license for free, upon request, so you will indeed be able to use both suites - for Silverlight 3 and for Silverlight 4.
the Telerik team
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system and vote to affect the priority of the items