Multiple Expander have to collapse if ONE is expanded

Question

Having 4 Expander controls. When one expander is expanded how can I make all others collapse/close?

Answer 1

@wassim-azirar asked to the accepted answer:

How can I expand 'Expander 1' at the application Startup ?

I added in the ViewModel:

SelectedExpander = "1";

Because of the fact, that the "1" is not the same object as the "1" in XAML this will not work, so I changed decyclone's answer like this:

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
    return (string)value == (string)parameter;
}

The answer of decyclone was very helpful for me - Thanks.
So I would like to share my experience if someone needs it.