A game has to be fairly unbearable for me to not want to finish it. Well, congratulations Sonic 4, you've landed in that small list of games I just straight up hate.
The parts that broke me? Lots of them, but most specifically the Oil world part 2: the Oil world has a mechanci where you run across sinking sand and have to jump in order to not sink into it and die, right? Fine, that's fair. At a certain point in the stage I was then forced to run along this sand, but in a low corridor where while on top of the sand Sonic's head would be touching the roof of the corridor. This immediately slowed my progress to a grinding halt. It gets better, though, because as you get further down the corridor, the roof is raised BUT Spikes are placed at the bottom of the roof! The spike tips reach exactly where the ceiling of the corridor was before, so if you jump too high you just die. This isn't ok but I could have at least gotten through the area if it wasn't very, very long or if there had been any rings whatsoever in it, but no, I got hit once, all of my rings disappeared in the sand, and then I had to go through the rest of this insanely long corridor without any protection. ♥♥♥♥ that.
The boss of the Winter world -- really cool looking level by the way -- was also full of ♥♥♥♥♥♥♥♥. Alright, cool, I'm racing Metal Sonic and I have to catch up with him and hit him, awesome! This is awesome! Wait, two minutes into the fight -- seemed like a long time for a sonic game -- he just hits the platform that I'm running on, breaking it. I immediately ran into the hole and fell to my death, restarting the fight. Alright, whatever, that was really ♥♥♥♥♥♥♥♥ because I didn't have the time to react, and it's dumb that I have to restart the boss fight but at least I know that's coming now and it won't happen again. Oh wait no, he just does that attack so fast and it's right on top of where you're running that you're just ♥♥♥♥ed if you don't anticipate it.
Elsewhere in poor design land is anything to do with Tails. Alright, so you occassionally have to press a button to join up with Tails for a special ability. Cool. Except for, you know, these abilities being so limited in utility that the only times you use them are when the game halts your progress unless you use the abilities. Of course, you use them so infrequently that if you hit these parts, you won't know what to do, so the game has to put up a sign telling you which ability you need to use to progress. Hey game, if you think I can't figure something out because I'm not using it frequently enough then I'm pretty sure you designed these abilities poorly.
Tl;dr: there are some cool parts to the game but they usually get ♥♥♥♥ed up with ♥♥♥♥ty design, horrible enemy placement and just straight up terrible choices.
No, ♥♥♥♥ this game. It's not a good game and it's not a good sonic game. Just don't waste your time.