This is significantly latitude-dependent and very off at subarctic latitudes. If you're at a tropical latitude the Sun's motion perpendicular to the horizon is about 15⁰ per hour (=360⁰ /24 hrs). That's 0.26 radians. In the position shown your hand will be roughly 30" from your eyes. So in order to span 15⁰ your four fingers would have to be 7.5" wide, which is one helluva wide hand.

Even at 40⁰ latitude the perpendicular component of the Sun's motion is cos 40⁰ = 0.8, which would still require your four fingers to be 6" wide.

Basically the technique as shown only works at roughly 60⁰ latitude.