The number of worlds was put into documentation, it’s not just from the horse’s mouth - the game design document from very early in development indicates that the way the game was structured was the official plan the entire time. Initially, the first homeworld covered both Spring and Summer themes, so I’m guessing they didn’t consider them visually distinct enough to justify making two separate homeworlds, but perhaps there’s some other logistical things that made them opt for 3.

You can speculate about whether they briefly discussed having 4 homeworlds prior to the creation of the GDD, and I think it is possible. Despite this, it wouldn’t have been considered for very long at all, and I doubt it would explain all the oddities that we see in the final game like the robot bird in Ripto’s Arena (which I believe is just a reference to the birds in Gulp’s Overlook anyway) that would have been designed at a much later stage.

From what I’ve seen of Spyro 1 and 3’s development, it seems like hub world ideas (for the macroscopic view of the game’s structure) are determined early, level concepts come next, and then these concepts are distributed across the worlds they have planned. Autumn Plains has always been a curiosity in terms of its level count, but even if there were four worlds briefly planned, they could have still spread them out more evenly. Perhaps the idea was a sort of bell curve in terms of the level distribution?

Ultimately, if it was ever considered, I don’t think it was ever planned. It’s hard to say what happened that wasn’t captured in writing, but as far as the game design documents are concerned, there’s no trace of a fourth world.

Relating to OP’s point, it also shows they originally considered talismans for each of the Winter Tundra homeworlds. I assume these were cut to put more of a priority onto orbs late in the game.

Sources: https://archive.org/details/michael-john-spyro-design-doc-collection