If you were disappointed with Sonic 4, consider checking these out for more successful attempts at making a non-handheld 2D platformer beyond Sonic 3 & Knuckles.
Sonic hacks are like Brawl mods, where they're tweaked versions of a given Genesis game. You'll need an emulator that can play Genesis games to play most of these these hacks.
Fangames are made from scratch, and as such, you can play them right after you download them, and how good they run depend on how good your computer is.
Sonic: Before the Sequel
A fangame that takes place between Sonic 1 & 2. Hence the name of this fangame. It has Sonic trying to stop the Death Egg from launching into space, and an alternate story of how Tails meets him and becomes his best friend.
What really made this fangame standout was its completely original & fantastic soundtrack from a group of dedicated fans. From what I know, it covers rock, jazz, orchestra, electronica, J-Pop, and even a bit well-done and intentionally-bad dubstep. Really, you gotta check it out.http://sonicbts.webs.com/ Zone 1 Act 1: https://www.youtube.com/watch?v=UC_R_IEXbUY YouTube Soundtrack Playlist: https://www.youtube.com/playlist?list=PL94E128D4184DFFA0
Sonic: After the Sequel
Made by the same guy who made "Before the Sequel", it's technically the sequel to it. Plotwise, it takes place after Sonic 2, again, hence the name, but before Sonic 3.
It details an extended plot of how the Death Egg crashes into Angel Island, and Sonic & Tails' adventure of preventing that from happening. You can now pick whether you want to play as Sonic or Tails, instead of having to automatically alternate between the two.
Like with Yoshi's Island shoutout in BtS, AtS has a Kirby shoutout through the use of the Mirror & Beam abilities.
Also, the soundtrack is as amazing as ever, probably even moreso than BtS. That's quite an accomplishment. Music & Gameplay Trailer: https://www.youtube.com/watch?v=mqnDlf5r8VU All of Zone 1: https://www.youtube.com/watch?v=HUPey_sT0rw YouTube Soundtrack Playlist
Sonic 1 Megamix
A hack that gives Sonic 1 an insanely huge overhaul. With completely redesigned levels, new moves, and you can play as either Sonic, Tails, Knuckles, Shadow, or Mighty the Armadillo, each of them have their own abilities, and all of them can turn Super. Zone 1 Act 1 (Version 1.0 to Version 4.0): https://www.youtube.com/watch?v=4APk--Ht4PQ Zone 2 Act 3 (Version 5.0) https://www.youtube.com/watch?v=YMQ8_hBFBEY
4.0 supports CD sound, but 5.0 will actually use it alongside the usual Genesis FM sound.
Sonic Classic Heroes
A hack of both Sonic 1 & 2 that brings together all of those two games' zones & Special Stages, puts in the physics and mechanics of Sonic 3 & Knuckles, such as the Hyper forms, and finally, giving the game a 2D version of the team system of Sonic Heroes. I always wanted to see this in a Genesis game and it surprisingly worked out.
When you finish all of the Sonic 1 zones, the ending will make a transition right into the Sonic 2 levels. Emerald Hill Zone w/ Team Super Sonic: https://www.youtube.com/watch?v=3wEdRy4_yt4
A very interesting hack of Sonic 1 where everything is explosive and difficult as hell. The creator of this hack claims himself to be the "Micheal Bay of Sonic games" because of this, as he narrates Sonic's painful quest through a parallel universe of Sonic 1's levels, while also occasionally scolding him for his performance.
Most of the levels in this hack aren't straightforward. Each of them ofter a slightly different experience from one another. Full Playthrough: https://www.youtube.com/watch?v=rfZKpgGjyaI
If you have the time and patience, download these games and see what you think of them, especially if you intend to compare them to some of the official games.