This review will be more of a justification of why I'm giving it a 2/5 rather than a 0.5/5. Let's get the actual review out of the way first. The game is playable, but the theme of the levels is extremely lazy and the gameplay is honestly boring. Splash Hill and Lost Labyrinth are just Green Hill and Labyrinth Zones from Sonic the Hedgehog (1991) while Casino Street and Mad Gear are just Casino Night and Metropolis Zones from Sonic the Hedgehog 2. Casino Night is really the only justifiable zone to steal from because Green Hill is overdone (though that's due to future titles after this) and both Labyrinth and Metropolis are some of the most hated Zones in Classic Sonic titles, which makes it more baffling that they ripped straight from those Zones instead of more loved Zones like Hydrocity (then again we don't want that to be tainted by this awful game). So far, this is the first mainline game to have a bad soundtrack. Every other game I've played so far from the very first to Unleashed had either an extraordinary soundtrack or just above average, but this game has basically nothing to it and the music gets annoying fast. Also, this game has no story, which you could argue is fine since it's a 2D platformer, but this is the sequel to Sonic 3 and Knuckles, which has very good visual storytelling, especially in the last part of the game with Hidden Palace and Sky Sanctuary. It just feels like the "Sonic 4" title doesn't really fit this game and was just tacked on to make money, which wouldn't be far from the truth since this game feels like a soulless cash grab. So why am I giving it a 2/5? Well, the quickest answer is: there's worse. When deciding a score, I base it off of other games in the rating and the ones above and below.
Edit: My opinions on most of the games that I said were worse have changed, so that part of the review didn't hold up well. However, while this game bores me, the rating I gave it basically means "it's playable start to finish, short enough that I could get through it and not care, but still frustrates me due to ____".
Now the thing that makes this game better than those is that the other games are much longer than this. While this game is pretty bad, at the very least it's an hour and fifteen at most as opposed to Shadow which was extremely long due to the developers expecting you to play ten times to get the True Ending. Here, you can just speed through and get to the end. You don't even need the Chaos Emeralds because nothing changes (unless you want to play as Super Sonic, then sure). I would honestly rather play a short and boring game than a long and tedious game with an unsatisfying ending. Overall, I would say while this would still be in the "do not play" category, I think the length of the game being extremely short makes this not as bad as other bad Sonic games, and you can quickly check this off your list if you plan to play it.
Edit: My opinions on most of the games that I said were worse have changed, so that part of the review didn't hold up well. However, while this game bores me, the rating I gave it basically means "it's playable start to finish, short enough that I could get through it and not care, but still frustrates me due to ____".
Now the thing that makes this game better than those is that the other games are much longer than this. While this game is pretty bad, at the very least it's an hour and fifteen at most as opposed to Shadow which was extremely long due to the developers expecting you to play ten times to get the True Ending. Here, you can just speed through and get to the end. You don't even need the Chaos Emeralds because nothing changes (unless you want to play as Super Sonic, then sure). I would honestly rather play a short and boring game than a long and tedious game with an unsatisfying ending. Overall, I would say while this would still be in the "do not play" category, I think the length of the game being extremely short makes this not as bad as other bad Sonic games, and you can quickly check this off your list if you plan to play it.