To quote: "The level recorded is Act 2 of the Mad Gear Zone level. Mad Gear Zone is based on Sonic 2's Metropolis Zone.
The seven minute long video is below for your delectation. It's obviously not going to be in super high-definition 1080p in fullscreen but it's fine (best watching it without fullscreening it since the video gets stretched & definition gets lost when Sonic is flinging himself around)."
Those interested can check out the leaked Sonic 4 Mad Gear Zone Act 2 Gameplay Footage below!