To go back to a point I started to make in the previous review and then stopped, let's talk about how I'm dividing up the eras of Sonic games. I'm of the general belief that paradigms shift in a gradual way that does not become obvious when looking for extreme granularity. Sonic, in my mind has three eras, but trying to define the when and where of those eras is tricky. To put in very evident signposts, very clearly: Sonic 1 is early era, Sonic Adventure is middle era, Sonic Unleashed is modern era.
Actually defining where the early era ends is the most difficult: Simply put, the pixel based Genesis/Mega Drive, Game Gear, SMS games are all very evidently early era. But Sonic gradually went into weird places, and while the Dreamcast is the middle era, the exact points around it I don't know.
The modern era, as I said elsewhere, lies with Sonic Unleashed. Everything I've read has stated that Unleashed is where they nailed down the 3d and got it working in a way used in Colours and Generations, and that's where I imagine it is going to stay. At least for the games Sonic Team's repulsive inability to stick to a working model doesn't rear its head.
It's very true, however, having played Sonic Adventure 2 (and hating it, yes) that Sonic's 3D begins all the way back there in some capacity. You can feel the outlines of the new ideas taking shape, but it's not there yet, and that's why I'm defining it like that. The middle era is when Sonic didn't "get" 3D, but it's still Sonic, I guess. Sonic Heroes was better about this, but it's still not quite Generations.

Meanwhile, Sonic actually has quite a few 2D games. Rush, Advance, so on and so forth. Many of these were developed by DIMPS, and while I don't personally like the DIMPS games, they went to them for the atavistic throwback that is Sonic 4 Episode 1, a modern era game desperately trying to be an early era game.
The results are... Interesting...
Sonic 4 Episode 1 is the sixth of nine games in the Summer of Sonic! The previous main entry is
here, while the next bonus entry is
here while the next main entry is
here. Those two won't be up open initial posting, but that's just how this works.