Returns the ISO 15924 alpha-4 code defining the script. Input can be a three-digit numeric or four-letter code Aaaa.
Definitions present: iso15924/codelists (esp [1], published 2021-01-25) lists exactly 208 alpha-4 id's, including Qaaa and Qabx, counting two. This enwiki template adds one line (into 209 then), to allow Qaaa–Qabx as a range of 50 codes, as intended by ISO.