What effect, if anything, does putting a slash in from of the field bound to the control, have? For example:
<telerik:RadMaskedNumericInput Grid.Column="1" HorizontalAlignment="Left" Grid.Row="2" Margin="5,0,0,0" VerticalAlignment="Bottom" Mask="####" FontFamily="Century Gothic" FontSize="16" Value="{Binding /ProbationStartYear}" IsClearButtonVisible="False" AutoFillNumberGroupSeparators="False" />