Function average_z4_ordering_depth 

pub fn average_z4_ordering_depth(depths: [u16; 4], zsf4: i16) -> u16

Compute AVSZ4’s saturated OTZ result from four cached projected depths.

This is the four-vertex counterpart to average_z3_ordering_depth and matches the GTE’s AVSZ4 operation with the supplied ZSF4 value. MIPS render loops should normally prefer average_cached_z4.