This falls into that problem of Sonic 4 being in an awkward position for being classic and modern at the same time. But as far as it goes, Sonic 4 is the transition to Modern Sonic, while Sonic Mania simply doesn't belong in that timeline, being an adventure exclusive to Classic Sonic. So by that logic, Sonic 4 and Sonic Mania can't (or shouldn't) be on the same timeline. Also, Sonic 4's description says: "Sonic the Hedgehog 4 Episode I picks up right where Sonic and Knuckles™ left off". If this is to be followed to the letter, then not only Sonic Mania but also Knuckles Chaotix could not precede Sonic 4.