While Sonic 4 completely lacks any originality, the core game itself is very solid and most importantly, fun.
I'll be honest, I have no idea what the hell the story is about. There are no cutscenes or dialogue in this game, but I think that's a good thing. I find that story elements and cutscenes in platforming games such as this, distract players from the overall enjoyment and pacing of the game.
The game plays a lot like the older titles in the series. There are 4 zones, with 3 levels in each zone, with a boss fight at the end of each zone. After you beat the first act, or level, of this game, it automatically unlocks all of the other acts in all of the other zones. This takes away the linear story progression of this game which I liked a lot in the original Genesis games. One thing I am kind of disappointing with this game is the lack of the originality. When people asked for a 2-D Sonic game that was like the old Genesis games, Sega really took that advice literally, and unfortunately, I'm here to tell you that's its for the worst. Almost every element of this game is borrowed from the first two games in the series. From the level design, to the bosses, and even to the special stage, I feel like I'm playing Sonic's greatest hits with an added homing attack. Oh, I forgot to mention, they added a new homing attack which is found in the newer 3-D Sonic games. Luckily, this addition is for the better, as it is very useful and adds more speed to the game. Anyway, the main objective of this game is to get to point A to B by running, jumping, swinging, gliding, falling, swimming, you name it. You collect rings along the way, and collecting 100 scores you an extra life. That's another thing I'd like to mention, they hand out extra lives in this game like candy. You'll find yourself getting 99 lives in about a half an hour of playing. Now is this a bad thing? I say yes, in fact, it's game breaking. It makes the game way too easy. I know Sega wanted to gear this game to the new generation of Sonic players, but wasn't this game made for the fans of the original games which were difficult but fair at the same time? Anyway, about the difficulty of the game, its pretty easy even without all of the extra lives. The only time the game gets difficult, is when it gets cheap. Luckily, its not that often, but the last two bosses of the game can get pretty frustrating. One thing I do really like about this game is the varied level design. Though, all of the zone's themes are borrowed from the first two games, they're still really fun to play. I'd also like to mention the special stages. If you complete a stage with 50 rings, you'll get the chance to play a special stage. These are pretty fun and the controls are pretty good surprisingly. The goal of these stages are to collect the chaos emerald at the end of the stage. The design is practically the same as the first games, but instead in this one, you tilt the stage as Sonic continuously falls down. They're challenging, but fun, and offer a nice change of pace with the game. Collect all 7 Chaos Emeralds and...well you can guess what happens. The biggest problem with this game is that it's way too short. I know they're planning on releasing an episode 2, but its still way too short for the $15 you pay for it. There are also leader boards to see who can get the best time and score in a level, but unfortunately, they have become victims to hackers, who completely ruin the competitive aspect of the game.
The graphics are gorgeous looking with fluid animation and great looking levels. The sound is a lot like the older games with all of the SE's the same as the older games, which I like. The music is great too.
Overall, the game is good but unfortunately, it's not worth the $15 you're going to spend on it, and it lacks originality.