Re: Back in my day…
Expert Systems - aka XPS - were quietly dissolved into online diagnostic systems and then mainly devolved into Vaguely Trained Systems as people couldn't be bothered actually working out what all that Bayesian nonsense is about. There are a few "proper" XPS around, diagnosing Important Things that Cost Money, but the term itself seems to be rarely heard[1]. Shame.
As for 4GLs - aside from the utterly, utterly stupid name[3], these were no more interesting than dBase - literally, just whatever "simple to use database language" someone wanted to flog you. What happened to them? Well, Sqlite allows everyone access[4] to the query language that actually *works* (not pretty, or easy, but in the end we all come back to it).
[1] which just means that people aren't looking up the info about old systems, MYCIN et al, and all the things we learnt from them. As we continually complain happens with - every other tech area, from basic coding onwards [2]
[2] lawn, off, now
[3] Sod what Wikipedia[5] says, the flurry of books and seminars etc about "4GL", spelt that way, in the 1980s was due solely to trying to jump on the hype wagon from the Japanese "Faith Generation Computer" research - this was for the fifth generation of *hardware*, not software: the intent was to build hardware that could run logic programming languages really fast, such as Prolog. Giving the slogan "Prolog, the Language of the Fifth Generation". Promptly misread by the flacks, who tried to find out what Prolog does ("oh, it uses a database of logical relations and resolves those to answer a query") - not understanding what a Horne Clause is nor what the Resolution algorithm actually does, they just heard "database". Ooh, we've got a database language but we can't claim it is Prolog, so, well, Prolog is a 5GL, what we have must be a 4GL! In the words of the great Welsh hard, "I know, because I was there!" and we pissed ourselves laughing at first then got pissed off trying to fend away the drivel merchants into the '90s. At which point, that AI bubble had burst and it was all very embarrassing, never mention it again.
[4] no, not Access, but I bet you can find some advert calling Access a 4GL
[5] that article is an incredible exercise in trying to force one specific viewpoint as "The One True Reality", just to boost the claim that the "4GL"s were actually anything interesting. The naming of generations of software languages is a bit convoluted, but back when we were studying languages, in the late 70s and early 80s before the "4GL" bubble, we were well past gen 4 languages, counting along multiple paths. For example, (1) machine code (2) assembler (3) macro assembler (4) simple compiled languages, e.g. FORTRAN (5) Functions as First Class Types, e.g LISP 1.2 (6) Object Oriented Languages, e.g. Simula-67. Which gets us all the way up to 1967, so from there to languages like Algol-W or even, gosh, Prolog and it's friends (let alone weighted logic languages, Fuzzy Logic languages...) we are *way* past "generation 4".[6]
[6] lookie here: if we follow *this* trail in the development of PLs, we find someone who came up with a novel way of doing things, so clearly this was the fourth generation of languages. But he was restrained by the filing system on the computer, which only allowed short file names, so he had to call the new language "FORTH".