What's the cross ratio of 4 planes?

Question

For example,
$H_1 : 3X_0 + 2X_3 = 0\\H_2 : −X_0 − X_1 + X_2 = 0\\H_3 : 4X_0 + X_1 − X_2 + 2X_3 = 0\\H_4 : 5X_0 + 2X_1 − 2X_2 + 2X_3 = 0$

Answer 1

Assuming they have a common line, find the normals of the planes and treat them as if they are a pencil of lines. Compute the cross ratio of those lines (using the usual ratio of sines of angles). Or, for fun, compute a ratio of cross products.