It is a sequel to Sonic 3 in name only. No one considers it an actual sequel. There is a reason Mania was called "the real Sonic 4". It's like how no one considers Federation Force to be a Metroid Prime game, even if it's branded as one. In fact FF is a perfect comparison because it too had a critically-acclaimed title slapped onto it to grab sales and only resulted in people hating even more for spitting on the name of a beloved series. Sonic 4 is legally a sequel to Sonic 3. it sure as fuck ain't a cultural sequel to it. Culturally, the entire Sonic fandom has near-unanimously repudiated it. That's what matters far more than its name, because in the end, a game lives or dies by how the culture it was designed for reacts to it. Our reaction has been near-unanimous disownment.