Sega, during the early to mid nineties, were on fire with their main series Sonic the Hedgehog games. Sonic the Hedgehog 1, 2 and 3 (and Knuckles) sold very well and all were considered instant classics at the time, and still considered to be classics even now. While Sega were busy creating these main series games and polishing them to be as fine as they could be, there were an awful lot of other Sonic the Hedgehog related games that were released around the same time, to placate the fans and to generate as much lovely money as they could. Come the invention of three dimensions in video games however, Sega found themselves in difficulties. A false start with their home console, the Sega Saturn, led to no new main series Sonic the Hedgehog games being created for it, instead their ideas being carried over and focused on their newer machine, the Dreamcast. This eventually resulted in the first fully 3D platforming Sonic the Hedgehog game, Sonic Adventure.
As time went on and Sega continued to create their 3D based Sonic games, the original sprite based games were placed in the back of peoples minds. Hardcore fans wanted a return to that style of gameplay, but Sega were adamant that the future of video gaming didn't consist of retreading old ground, and so continued to try and invent new gimmicks for their 3D based games. Eventually in 2010, Sega decided to compromise and try to combine both styles of games into one, combining the side on camera perspective notable of the early nineties games with 3D models and assets. They boldly decided to call this game Sonic the Hedgehog 4, to tie it into the original three games and to try to appease the fans of those games.
Sonic the Hedgehog 4 was to be released in three parts (or episodes) that come together to form one game. They had tried a similar tactic with the release of Sonic 3 and Knuckles, but this gambit was different. Each episode wasn't the size of a full game, only having three levels in each, and so weren't sold as full games on release, having a budget price. The three pieces were to be played in order to simulate a full game, or played by themselves for bitesized portions of platform gaming.
Unfortunately for Sega, their plan didn't work out the way they wanted it to. To start, only two of the three planned episodes were ever released, leaving this game literally unfinished. Playing through both parts as they are, something feels off about the game. Sonic had appeared in previous games with a 3D model with a 2D perspective on the Nintendo DS with less issues, but something about this game was off.
The 3D models on a 2D plane idea just didn't work as well as the sprite based games of old. Sonic didn't handle properly, he felt floaty and imprecise. In sections where you have to cut the speed out and perform tricky platforming sections, Sonic wouldn't react the way you wanted him to, and this often led to unintentional deaths. Precise controls mixed with extreme speed were something the older Sonic games managed to pull off quite well, and it's a trick the 3D modeled Sonic never managed to get 100% right. Sonic's speed is off in this game as well, taking time to fully get into gear. The controls never feel like they are finalised and ready to go, which is strange for a released game from a company like Sega; it should be polished instead of feeling like they are half finished.
Finally there comes a Sonic the Hedgehog game where the music is not immediately memorable or catchy. The streak has been broken, the tradition of classic bouncy music that infectiously worms its way into your head is gone. You could hum a few bars of any song from Sonic 1-3 (and Knuckles) and you could find gamers to hum along with you. Good luck trying to remember any of the songs from Sonic 4 to hum. They don't have the same bombasticity that the originals had.
There are a lot of problems with releasing a new game in a popular series sixteen years after the last game in said series. In all that time, nostalgia plays a factor, adding a rose tint to the older games and making them seem superior to anything that could be created nowadays. Sonic the Hedgehog 4 was probably never going to be as widely recognised and loved as the games that came before it, which must have put a lot of pressure on the company who developed it, Dimps. By this point, Sega were long gone from actually creating Sonic games themselves. Perhaps if they had stuck with it, this game might have turned out differently.
Not groundbreaking, not revolutionary, but worst of all not memorable. Sonic the Hedgehog 4 falls at the first hurdle and makes the most cardinal of sins; being bland beyond belief. It's just not memorable, it is wholly forgettable. It is not necessarily a bad game, although it is an incomplete one. It's a game that will barely be remembered, a footnote in Sonic the Hedgehog history despite being numbered as a main series game. Unlike the others, this is nothing to get excited about. Not a terrible game, but one that feels incomplete.