Plot a 2 by 2 comparison of data objects. Plotting parameters and camera
    positions will all be the same.

def plot_compare_four(data_a, data_b, data_c, data_d, disply_kwargs=None,
                      plotter_kwargs=None, show_kwargs=None, screenshot=None,
                      camera_position=None, outline=None, outline_color='k',
                      labels=('A', 'B', 'C', 'D')):
    """Plot a 2 by 2 comparison of data objects. Plotting parameters and camera
    positions will all be the same.
    """
    datasets = [[data_a, data_b], [data_c, data_d]]
    labels = [labels[0:2], labels[2:4]]

    if plotter_kwargs is None:
        plotter_kwargs = {}
    if disply_kwargs is None:
        disply_kwargs = {}
    if show_kwargs is None:
        show_kwargs = {}

    p = vtki.Plotter(shape=(2,2), **plotter_kwargs)

    for i in range(2):
        for j in range(2):
            p.subplot(i, j)
            p.add_mesh(datasets[i][j], **disply_kwargs)
            p.add_text(labels[i][j])
            if is_vtki_obj(outline):
                p.add_mesh(outline, color=outline_color)
            if camera_position is not None:
                p.camera_position = camera_position

    return p.show(screenshot=screenshot, **show_kwargs)