Was World War One known as that before World War Two? I'd always heard it was called "The Great War"- so wouldn't the POD be after WW II?
I think the problem, frankly, is that WW I was within living memory and the others weren't. So, your POD would probably have to be...you know, "French and Indian War" is kind of dumb since IIRC they were Allies. So, maybe that's the best war to start calling a World War. Maybe nobody could think of what to call it.
So, say some historians and writers start calling The F&IW WW One, and some smart guy comes back and says, "Look, the War of Spanish Succession was fought all over the world (at least, I'm gessing it was, I don't know much about it), just like this latest war; so this should be World War Two." And, thus it's named.
As for your original query, I think you'd get the napoleonic Wars called "the war to end war" in the 1800s, with the Congress of Vienna set to create a "new world.' The 18th century (which is when they'd say it started, tecnically, though most of it was fought in the 19th) would be seen as the "era of great wars," and Europe would be seen as having begun a new era of peace.
I'm not sure what that would do; but most people would figure they were "too civilized for another 'World War,'" with the "barbarity of the world thrown into a war something reserved for our ancestors." When so much of the world got involved, it would be a huge shock.
Probably about as big of a shock as OTL; the only difference being that some psychologist, with there having been 4 World Wars, might in the interwar period come up with some hypothesis of mankind viewing war like an alcoholic views booze, as a terrible addiction that must somehow be cured. Whereas...well, I'm not sure what they viewed as the reasons for war then as it was, but I'm pretty sure the term "addiction" never occurred to people then.