Unable to answer "What diameter is specified on the sign for this fire hydrant?" – Underground Fire Hydrant – https://osm.org/node/10101444827 via StreetComplete 62.0:

Sign says 0 and 4 but streetcomplete suggests this is incorrect

Attached photo(s):
https://streetcomplete.app/p/317518.jpg
https://streetcomplete.app/p/317519.jpg