class SplitToFour
{
    public static readonly DependencyProperty FirstProperty =
           DependencyProperty.Register(
               "First",
               typeof(string),
               typeof(SplitToFour),
               new PropertyMetadata((d, e) =>
                     { ((SplitToFour)d).UpdateSource(); })));

    // the same for other three

    public SplitToFour(DependencyObject o, DependencyProperty p)
    {
        DependencyPropertyDescriptor dpd =
            DependencyPropertyDescriptor.FromProperty(p, p.OwnerType));
        dpd.AddValueChanged(o, delegate { UpdateParts(); });
    }
}