About a month ago several videos purporting to show gameplay from Sonic the Hedgehog 4 leaked out. This one, visually consistent with the others, shows a new stage: "Mad Gear Zone."
YouTube user SuperBlizHedgehogRev got these up on Friday and they've managed not to get smashed down by a Sega copyright claim since then. Skip ahead to 3:14 to see him turn into Super Sonic, whose power can only be stopped by a phenomenon known as The Bottom of the Screen.
Update: The video was taken down by YouTube. Above is a three-minute excerpt from it.