The California Residential Code (CRC) 2019 has a table of header sizes for various spans / building widths / roof loads: https://up.codes/viewer/california/irc-2018/chapter/6/wall-construction#R602.7
size column list various combinations of double, triple and quadruple 2x materials (e.g. 2x8, 2x10, 2x12).
Why are there no 4x options (e.g. 4x8, 4x10, 4x12)? Does a different section of code cover that, and do they have different allowable spans?
Context: my local building department suggested I revise my plans from 'double 2x12' to 'single 4x12', and said "like the CRC requires". I'd like to comply with their wishes, but I'd like to fully understand them by reading them in the code. I'd prefer to use double 2x, as it's easier to get in place.