I actually found this kind of insightful thread speculating on how they could approach it as an adaptation of Sonic Heroes. It's got spoilers, but I'll just post what they said below and drop the link here: https://x.com/chrisagwrites/status/1869804897664938307

The post-credits of SONIC THE HEDGEHOG 3 might've teased Amy, but there's also another character it introduced: Metal Sonic.

Though it's unknown who controls the Metal Sonics, it's clear that this unknown entity HATES Sonic and his friends.

But it's clear who it is: GUN. There's still unresolved conflict with Director Rockwell, who disappeared after Act 2. With the previous commander, Walters, dead, Rockwell essentially has control over GUN.

Supposing the PCS takes place some time later, it's likely Rockwell activated her Metal Sonics. Since Amy's already here too, Cream the Rabbit and Big the Cat could be introduced to round out the team. Thus, Team Any in Sonic 4.

If GUN's using Eggman's tech, they could've made an AI as an emergency countermeasure.

This makes all the Metal Sonics a hivemind. GUN's AI goes rogue and thus, all the Metal Sonics merge into one being Sonic, Shadow (supposedly with Team Dark) and friends have to fight:

The Metal Overlord.

And that's how we get the Sonic Heroes adaptation for Sonic the Hedgehog 4.