I can't even describe how brilliant every single bit of this hack is. The level art is absolutely brilliant, pushing the Genesis video hardware to its limits and then some, proving that you don't need millions of colors to create something visually pleasing. The environments are tride and true to the Sonic formula, fitting the common level cliches and even more obscure ones.
The level design is extremely well thought out, taking advantage of all the unique characters' abilities—Sonic, Tails, and Knuckles, respectively—in multiple branching paths, well balanced gameplay, and containing many well thought-out level gimmicks. Most hacks I see have some pretty solid level design, but seldom does it make use of all the gimmicks like the original games did—spinning platforms, collapsing platforms, strategic enemy placement, you name it. This is what really makes the Sonic experience authentic.
I'd comment on how well the music fits, but of course, I was the one who did it (=P). I'll explain my reasoning, though; see, when I played through this hack, I scoured through my SMPS VGM collection on a case by case basis with the counsel of a friend of mine, DJ Squarewave. I would propose a level theme and we would explore the SMPS library trying to find songs that would fit the levels perfectly. The first level's music was my choice (I had previously used it in Megamix), as was Garnet Garden and Robotic Ruins' music. Dusty Desert was a recommendation from DJSW, Roof Top was something me and Rika mutually agreed on (it took a lot of manual editing of the source tune to get it to loop right), as was Crystal Cavern (after all, who can go wrong with Ristar? =P). The invincibility music was chosen in the same reign—I found it interesting particularly that it used a very similar chord structure to a lot of Sonic invincibility music, so it fit the bill nicely. The boss theme was just plain awesome.
The presentation here, overall, is of the utmost quality. You actually feel like you're playing a legitimate Sonic game, and while most hacks are extremely fun to play, they simply don't even live up to the calibur and standard of quality that Sonic 2: Advanced Edit sets. I wouldn't even place Megamix that high up—it wouldn't be fair. There was an immense amount of effort and skill put towards to creation of this hack, and for that I diligently commend Rika for his hard work. If anything could truly be called Sonic 4, I'd give that title to this hack.
It's an immense, crying shame that this hack has to go down the drain for something as mundane as Sonic 4. I'll contribute to the project every so often, sure, but in no way does it ever deserve to overshadow this magnificent piece of work. I'll do my best to not let the effort put into this hack be in vain.