The RadDatePicker has not been designed to work that way. However, if you need the user to choose a year only (which is a number), you can use:
RadMaskedTextBox with a 4-digit mask
the Telerik team
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?
Telerik Public Issue Tracking
system and vote to affect the priority of the items