Thank you for contacting us.
After Q1 2014 version, our UI for Silverlight
suite supports only Silverlight 5. So, the answer is yes. It's better to recommend your users to install Silverlight 5 version in order to run your application properly on their machine.
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Feedback Portal
and vote to affect the priority of the items