Linked Properties in a ViewModel

public class ViewModel1 : ViewModelBase
{
    private int _foo;

    public int Foo
    {
        get { return _foo; }
        set
        {
            if(value == _foo) return;

            _foo = value;

            OnPropertyChanged("Foo");
            OnPropertyChanged("Bar");
        }
    }

    public int Bar
    {
        get { return _foo * _foo; }
    }
}

public class ViewModel2 : ViewModelBase
{
    public ViewModel2()
    {
        PropertyChanged += ViewModel2_PropertyChanged;
    }

    private void ViewModel2_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "Foo")
            OnPropertyChanged("Bar");
    }

    private int _foo;

    public int Foo
    {
        get { return _foo; }
        set
        {
            if(value == _foo) return;

            _foo = value;

            OnPropertyChanged("Foo");
        }
    }

    public int Bar
    {
        get { return _foo * _foo; }
    }
}

public class ViewModel3 : ViewModelBase
{
    private int _foo;

    public int Foo
    {
        get { return _foo; }
        set
        {
            if(value == _foo) return;

            _foo = value;

            OnPropertyChanged("Foo");
            UpdateBar();
        }
    }

    private int _bar;

    public int Bar
    {
        get { return _bar; }
        private set
        {
            if(value == _bar) return;

            _bar = value;

            OnPropertyChanged("Bar");
        }
    }

    private void UpdateBar()
    {
        Bar = _foo * _foo;
    }
}

public class ViewModel4 : ViewModelBase
{
    public ViewModel4()
    {
        PropertyChanged += ViewModel4_PropertyChanged;
    }

    private void ViewModel4_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "Foo")
            UpdateBar();
    }

    private int _foo;

    public int Foo
    {
        get { return _foo; }
        set
        {
            if(value == _foo) return;

            _foo = value;

            OnPropertyChanged("Foo");
        }
    }

    private int _bar;

    public int Bar
    {
        get { return _bar; }
        private set
        {
            if(value == _bar) return;

            _bar = value;

            OnPropertyChanged("Bar");
        }
    }

    private void UpdateBar()
    {
        Bar = _foo * _foo;
    }
}