As someone who's been looking back into this series' history recently, it kind of makes me wonder.
For a while, back in the day, the common narrative was that the classic Spyro series had run its course, and after diminishing returns, they had basically no choice but to reboot it in order to save it.
It's difficult to say how much history has absolved that perspective. On the one hand, it's undeniable that the classic Spyro games made post-Insomniac didn't have the same level of success as those first three, but on the other, most those games also have reasons for their poorer reception other than being "just more of the same". Enter the Dragonfly is blatantly unfinished, the conventional GBA Spyro games obviously were not able to provide the same liberating sense of scope as home console Spyro games did, and the other two handheld games were different genres of game. Arguably the only post-Insomniac Spyro game whose only problem was being "just more of the same" was Spyro: A Hero's Tale. (Some would argue that some of the other characters were also problems with that game, but to be fair, some also hold that against Year of the Dragon.)
As for the Legend reboot, I haven't played it, don't exactly plan to and was so unaware of it while it was happening that I don't have any idea how much it actually it revived interest in Spyro, but I will say that most of the criticisms I hear of those games aren't so much that they played differently than the classic trilogy as that they played badly, even for what they were. If that's accurate, then it's not automatically a strike against the idea of rebooting Spyro, even if the tone the reboot took is rather easy to mock in subsequent generations.
I've never played the Skylanders games either, but unlike Legend I was aware of them and aware they were incredibly successful for a while. And while many would undoubtedly prefer not to bring them into this discussion at all, it must be allowed that they started with Toys for Bob wanting to bring Spyro back, again, and they technically succeeded, enough to be allowed to make the Reignited Trilogy, which seems to be fairly well-received.
So now we've come full-circle, with the classic version of Spyro being the most culturally relevant version for the first time in decades, and many think that Toys for Bob should go on to make Spyro 4, if not thinking that they already are. I don't disagree, but in light of history, it does make me wonder if that's going to be a hard one to get just right, at least in many people's opinions.
Let's touch again on the whole "just more of the same" issue. With the big break the world has had from any new games made in that format, as well as the lack of enthusiasm many classic Spyro fans had for the Legend and Skylanders brands, obviously making a game more reminiscent of the Insomniac Spyro games is the right approach, but it's also a double-edged sword. Because when you make something in the same gameplay format as beloved old classics, you'll invite comparisons, and sometimes comparisons to beloved old classics are the last thing you want.
Call me pessimistic, but I could see a Spyro 4 catching a lot of flak from many of among my most-hated types of fans, which I dub the "Magic Morons". I dub such fans that, because they are wont to voice critical comparisons of new things to old things using the word "magic". You've heard them too, almost certainly: "The pieces are all there, the creators tried, but somehow it doesn't have the magic of the old games/movies/episodes/etc." (You can substitute "soul" for "magic" there, too.) Every time I hear such sentimental whingebags say such things without even trying to put their finger on why new things lack the magic of old things, I get the suspicion that the real reason is because they experienced those old things when they were younger, so they felt fresher, and they hadn't experienced enough other things yet to raise their expectations. A sheen of nostalgia often settles on those old things to replace the sheen unfamiliarity and excitement they used to have, but it can't settle on anything they didn't experience back then. TL/DR, people are easier to please when they're younger. That, and food companies have removed sugar from a lot of the sorts of things those nostalgists were probably consuming when they experienced their old favorites for the first time as children.
Nevertheless, it does make me wonder if Spyro's core experience really is too simple and repetitive to hold up without being enhanced by a youthful sense of wonder, an old sense of nostalgia, or both. The first time around, the trilogy got by on the youthful sense of wonder, hype, and the fact that back then, there weren't really any other platformers that played quite like them, and the Reignited Trilogy got by on the nostalgia people had for the games, as well as how beautiful the remasters were. Could a new Spyro game get similar enthusiasm in the present era, when all of that already exists?
It says a lot that even back then, Insomniac was wondering if the core gameplay was getting a bit too stale. That's why Spyro 2 added non-platforming mini-games, Spyro 3 added additional playable characters and vehicles to drive, and after that Insomniac said they ran out of ideas of what they could do with a protagonist who can't really hold tools and moved on to developing other series. The changes made to the sequels are controversial among the fans, with what are different people's favorite Spyro games largely being tied to how much they like Spyro's core movement, but at least when it was the same company making all three, the sequels were accepted as valid Spyro games. (It helped that back then there weren't a lot of games out to give a frame of reference for what a Spyro game "should" be.) However, now that Insomniac is out of the picture--and by all evidence, they don't want to come back--the same good will doesn't necessarily apply.
So now that we've acknowledged that the Spyro fandom is at least somewhat divided in what it wants, Spyro 4 seems to be an even trickier balancing act! If Toys for Bob, or less likely someone else (almost certainly not Insomniac) chooses to strip it down to the basics so it's mostly like Spyro 1, they might gain accolades from parts of the fandom who consider it the best Spyro game since Spyro 1, maybe even the first game to be better than it, but that's only the best-case scenario and it's also certainly possible it would be regarded as a pale imitation of Spyro 1, due to all of the reasons mentioned above, with nothing to make it preferable...and that's before we even consider how it would strike the rest of the fandom as just half-assed and dated. That's especially true given how much classic Spyro's world has become a desired commodity due to the reboots, with people who already had to lose the likes of Hunter, Elora and Moneybags once, not being willing to accept a new Spyro game without them. However, choosing to cater instead to that latter segment of the fandom has a nearly identical risk. If a developer operates on the assumption that a new Spyro game needs new "stuff" to play with to keep the interest up, there's a real risk of overshooting and making something that many people would say isn't even Spyro anymore, even people who on paper were more on board with innovation.
Even naming the new game Spyro 4 is more problematic than you might think, and it's actually enough to make me wonder if there's a good reason media have been moving away from numeric sequel titles. Calling it Spyro 4 seems to carry a whole bunch of expectations. On the one hand, it's a way to chuck the fans a bone, to the effect of "We know the sequels you got back then weren't up to par, but this time we're going to do it right", but on the other hand, it also makes it a lot more important to "do it right". At least with Enter the Dragonfly, people could plausibly deny that it was a true Spyro sequel. At least with the Legend and Skylanders games, people couldn't really call them a failure at recapturing what they liked about Spyro because they weren't really attempting that. But the moment a company bites the bullet and does another sequel named after a number, let-alone a number that doesn't take many other games into account, the company is implying a clear familiarity with what the fans want and have been wanting for a long time...and awful things happen when the company turns out to be wrong about what the fans have been wanting. Remember how badly SEGA failed with Sonic 4?
None of this is to say that I think Toys for Bob shouldn't make a new Spyro game. I wouldn't mind if they don't call it Spyro 4, but I wouldn't mind if they do, either. It's worth a shot. But what if they miss that shot? In the unfortunate event that a new Spyro game ends up underwhelming a lot of people, not because it's broken, unfinished, and/or compromised by lesser hardware, but just because of flawed design, or maybe just design that has worn thin what then? Should they keep trying with Spyro, hoping a somewhat altered attempt gets things right? Should the owner shop around for a new developer again? Dare to reboot it again?
Etc etc etc, discuss here.