NFPA 13 (2010 ed) says in 126.96.36.199.3.2, "Where soffits used for the installation of sidewall sprinklers are less than or equal to 8 in. (203 mm) in width or projection from the wall, additional sprinklers shall not be required below the soffit when the sidewall sprinkler on the soffit is within 4 in. (102 mm) from the bottom of the soffit."
That's very clear when the bottom of the soffit is horizontal and parallel to the floor. The annex entry for 188.8.131.52.3.2 illustrates exactly that: a square soffit with the sidewall sprinkler up to 4" from the bottom of the soffit. The 4" measurement is shown on the vertical surface.
My question is, when the bottom of the soffit is angled, as shown below, where is the 4" measurement taken: form the junction of the bottom of the soffit and the vertical surface, or where the bottom meets the wall?
In the case that sparks this question, the angled piece is concealing an air pressure line, leaving no room for additional pendent sprinkler heads under the soffit