Hi Bhavik,
The project will compile with older assemblies, however there will be a lot of breaking changes, especially with Silverlight 4. So, it is highly recommend to upgrade in order to compile your project against SL4 and benefit from its approvement. I can suggest you to use one of our latest assemblies, at best the Q3 2011 SP1 version.
Regards,
Konstantina
the Telerik team
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