Cannot seem to get Xamarin.Forms to work with WPF

I've followed the instructions here: https://docs.microsoft.com/en-us/xamarin/xamarin-forms/platform/other/wpf

But, I'm stuff on probably a simple problem: In my MainWindow.xaml.cs file, on this line:
LoadApplication(new App4.App());
I get the error:
Argument 1: cannot convert from 'App4.App'

This is becaue the "App" from App.xaml.cs is derived from type System.Windows.Application

Should I be changing app.xaml / app.xaml.cs ?