I would replace the ? in your = Sum ( ? ) expression with the entire Iif function from your detail row, e.g.
= Sum ( iif(Fields!Answer4.Value="Very",CInt(3),iif(Fields!Answer4.Value="Quite",CInt(2),iif(Fields!Answer4.Value="Slightly",CInt(1),iif(Fields!Answer4.Value="Not at all" ,CInt(0),"")))) )