Genesis/Mega drive says hi.
The Genesis/MegaDrive (and even the PC Engine/TG16, which released in Japan the year before) are both clearly Gen 4 consoles alongside the SNES.
The only consoles that arguably could get a generation all to themselves would maybe be some of the consoles released very late into the 2nd and 4th gensm which sort of were a gen 2.5 or 4.5 (Colecovision, Sega SG1000, Atari Jaguar, etc). But those are all massive outliers.
The Switch ironically is a candidate for getting a generation to itself (or more accurately, being some sort of Gen 8.5). Though I will say that generally Nintendo handhelds, since the days of the GBA, have been 2 generations behind in terms of power compared to home consoles so that's a strong argument for the Switch being Gen 9 proper. (Gen 6 GBA was as powerful as Gen 4 SNES; Gen 7 DS was as powerful as the Gen 5 PSX; Gen 8 3DS was as powerful as the Gen 6 Gamecube, and now Gen 9 Switch is as powerful as the Gen 7 PS3).
By these rules, we should expect Gen 10 to get started some time in 2023 or 2024 with a Switch 2 that will be somewhere about as powerful as a PS4 or PS4 Pro.