Unfortunately the migration process does not migrate any custom user controls that you have on your website. Such controls need to be rewritten in order to suit the new 4.x architecture. For that purpose you need to consult our API reference, which you can find in our SDK. Also in our SDK we have provided the templates for the built-in widgets we use in Sitefinity 4.x, so you may look at them as well.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items