Hi Tushar Gupta,
Our Silverlight 4 controls are the same controls that you are currently using - the difference is that our Silverlight 4 controls are build against the Silverlight 4 framework. While we do use some specific Silverlight 4 features to provide better experience (for example clipboard access) for our Silverlight 4 controls, their behavior is the same as their Silverlight 3 counterparts.
Upgrading to Silverlight 4 should not be a burden but the best way to determine if there are problems is to try to convert all of your projects to Silverlight 4 ones. If you do encounter problems we will try to be of much help as possible.
the Telerik team
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