Yeah, he does look slow. I'm hoping that that is just some sort of extra CG created to advertise, rather than straight out of the game.
You couldn't play Sonic 1 with Knuckles if you locked it in, it would come up with an error screen of Sonic and gang waving their fingers at you. BUT if you pressed A, B and C all at the same time, you were taken to a secret game where you could play the annoying Sonic 3 special stage for EVER.
Here's what some chump from SEGA has to say about Sonic 4:
Ken Balough: Sonic as a brand is definitely not moving away from our packaged console versions. Making Sonic the Hedgehog 4 Episode I for download is just a new way to offer a great Sonic experience through a different medium.