My memory of the whole "nGL" language thing was that it was more a marketing term than anything you would find in an academic paper. It was particularly common in the IBM world, for example in books[1] and advertisements[2]
On PC, I remember platforms such as dBase were often referred to as 4GL. In retrospect, it feels like was a short way to say "very high-level domain-specific language for databases".
[1] https://www.oldcomputerbooks.com/pages/books/R504/james-mart...
[2] I can't find a good example, but I remember seeing ads for "4GL" tools in Dr Dobbs regularly in the mid-90s.