My understanding is that OSHA defines the working length of the ladder as the length of the ladder from the foot of the ladder to the upper (or top) support:

1926.1053(b)(5)(i) Non-self-supporting ladders shall be used at

an angle such that the horizontal distance from the top support

to the foot of the ladder is approximately one-quarter of the

working length of the ladder (the distance along the ladder

between the foot and the top support).

The way it is usually shown is with the distance between the base of the ladder to the wall as "L/4", and the ladder rails from the feet to the upper support point as "L".

Your description above figure 91 is correct in the first sentence of the paragraph. But, the next sentence beginning with "For example,...) is incorrect because it says that "the base of the ladder should be one-quarter the height, or 4 ft away from the building." This is not correct. A correct statement would be "the base of the ladder should be one-quarter the working distance "L", or in this case 4 ft away from the building." If the "L= 16 ft" is correctly labeled as the real working length of the ladder (along the rails of the ladder up to the top support,) and not the height of the building.