public class ConverterToggleButton: IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value == null || parameter == null)
            return false;
        return ((string)value).Trim() == ((string)parameter).Trim();
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value == null || parameter == null)
            return false;
        if ((bool)value == true)
            return parameter;
        return "";
    }
}

<TextBox Value="{Binding myVar, Mode=TwoWay  }" x:Name="myText" Width="50" Height="30" Background="White"/>
        <RadioSwitchButton IsChecked="{Binding ElementName=myText, Path=Text, Converter={StaticResource ConverterToggleButton}, ConverterParameter='0'}"/>
        <RadioSwitchButton IsChecked="{Binding ElementName=myText, Path=Text, Converter={StaticResource ConverterToggleButton}, ConverterParameter='1'}"/>
        <RadioSwitchButton IsChecked="{Binding ElementName=myText, Path=Text, Converter={StaticResource ConverterToggleButton}, ConverterParameter='2'}"/>
        <RadioSwitchButton IsChecked="{Binding ElementName=myText, Path=Text, Converter={StaticResource ConverterToggleButton}, ConverterParameter='3'}"/>