4GL was really more a marketing slogan than a true generation. The idea was something like "with third generation tools, you have to drive the car, making every turn yourself, with 4th Gen., you say "Go to the Ritz!".

It wasn't true, although they did make some operations easier in tangible ways.

Rust is a wholly different kind of thing -- not easier than, say, Java, but lots more control with better guarantees. It's more a systems programming language. 4GLs were all application-focused.