I came across this nickname about the city of St. Louis a few weeks ago here https://en.wikipedia.org/wiki/List_of_city_nicknames_in_Missouri
I knew that St. Louis was the 4th largest city in America at one time, and finding that data is pretty easy. However, I haven't been able to find any more info on the nickname itself. I read the book "The Great Heart of The Republic" by Adam Arenson last year, and though I wasn't looking at the time, I don't recall any mention of the name in the book, which was all about how St. Louis was once poised to be one of the greatest cities in the country.
Does anyone have any information on when this term came to be, who coined it and if it was used widely or for a significant stretch of time?
It wasn't so much a nickname in the sense of, say "The Show-Me State," but more like an apositive sobriquet: "St. Louis, the fourth city." According to James Neal Primm in Lion of the Valley, the economic/political elite of various cities were quite competitive in the later 19th century, including over population. Journalists stirred up a minor frenzy, even scandal over the results of the 1870 census. St. Louis, concerned that the Civil War's upheaval had caused a drop in state and civic population, actually arranged to withhold census figures until after the other big cities--basically, Chicago--had released theirs. St. Louis came out ahead, and local newspapers made a big deal of it being the "fourth [largest] city."
If you're interested in the history of St. Louis, definitely pick up Primm. To my knowledge, it's the most comprehensive history of the city out there, mostly socio-economically oriented--it's the proverbial "drinking from a fire hose," very dense. The third edition runs into the 1980s, and the bibliography is great!