Wait a minuite, Wait a MINUITE, WAIT A MINUITE!
Sonic 4 can not be compared to this, cause it is like comparing Tim Burton's joker to Christopher Nolan's Joker, both directors potrayed the same character in a different light.
First of all this Video is based on Sonic 3 and the Knuckles expansion pack where as Sonic 4, is only Sonic 4 in name it is actually more closer to a sequel of the orignal Sonic or Sonic 2 in the asthetic sense.
Sonic 3 was all about realism, it had evolved out of the checkered textures of Green Hill Zone and Emerald Hill Zone to realistic looking lavish naturesque terrain and that is why it is looking so excellent.
Sonic 4 is all about the return of the checkered backgrounds, if you want a Sonic game to look like this, expect it from Colors or Unleashed 2 not Sonic 4.
That is all I have to say, I am not saying the video isn't jaw dropping I am say the art direction in both games is very different and Sonic 4 is 2DHD (right?).
All about different design choices really.