Interesting possibility, jools. I have always wondered myself.
I thought Mark Evanier's site might have some thoughts and this is all I could find was this statement -
"To pioneer this form, he devised the series that, for reasons that remain in dispute, came to be called the Fourth World. The characters, and a rough idea of their milieu, had come to him in his latter days at Marvel. You can see the embryonic stages in the last Tales of Asgard stories he did in the hindquarter of the Thor comic for a time. At the time, he was, as is well known, unhappy with his lot at Marvel. He had co-created some of the most popular and profitable characters in comic book history only to quickly lose both copyright and creative custody of them."
There was a link from ME's site to a site run by Sean Walsh - http://fastbak.tripod.com/ that had some FAQs - here is what is offered there from some folks. What you found certainly seems plausible
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"7. Why's it called 'the Fourth World?"
Watcher - Probably because of the modern habit of dividing the nations of Earth into First World, Second World, and Third World countries. Kirby denoted his characters and situations as coming from a different "world" than any of the others.
Mario - No one is really sure. Two common theories are: Jack meant it to be beyond the First, Second, and Third Worlds of then-modern politics, and another, noting that the phrase first appeared on issue for of the New Gods series, thinks that an overly excited copy writer meant it as "This is the Fourth World Jack Kirby has created in this series" and it just took off from there.
From the Jack Kirby FAQ by Bob Heer - "I've heard some creative explanations for the name "Fourth World" being applied to Kirby's greatest creations, the saga of the New Gods. An extrapolation from the "third world" term. Counting the early 40s superhero stuff as the first world, the later S&K stuff as the second, the Marvel Universe as the third. The fact that there were four books involved. A reference to the DC convention of naming parallel worlds (Earth-1, Earth-2 and Earth-3 existed by then). A reference to the Hopi creation myth of "Four Worlds" (my favourite).
"The truth seems to be that either someone in DC's production department put "Kirby's Fourth World" on the cover of the fourth issues of NEW GODS, MISTER MIRACLE and FOREVER PEOPLE (and Kirby's seventh issue of JIMMY OLSEN, which came out around the same time), or Kirby put it on the cover of one of those books and someone at DC picked up on it for the rest (Steve Sherman claims he heard Kirby use the term before it appeared in print, while Mark Evanier doesn't recall hearing it).
"Somehow the name stuck (perhaps because Kirby didn't really give the saga an overall name, and it was usually just called the tetralogy (or trilogy for those who whould deny JIMMY OLSEN his place in the sun)). The next usage I know of was in the letter page of MISTER MIRACLE #7 (there may have been a house ad in some DC books the month before, but I haven't tracked it down yet), in a reply by Evanier/Sherman. A very matter of fact usage, so the term seems to have caught on with fandom quickly. It appeared again on the cover of MISTER MIRACLE #10, and in letters in MISTER MIRACLE #16 and #18. The phrase also appears in a few pencils for KAMANDI and DEMON covers, which adds a whole other twist.
"So the explanation seems to be either 'don't mean nothing' or 'might mean something, but Kirby doesn't seem to have told anyone.'"