Soooooo maybe Sonic 4 isn't so new.   Depending on how you count things that is.  1UP has a article about this little gem to the left.  Seems back in the day, if you knew who to talk to you could have scored a Sonic 4 bootleg on the Super Nintendo.  The game is actually a hacked version of Speedy Gonzales: Los Gatos Bandidos.  In it Sonic rescues caged versions of Mario.  I'm sure if you know where to look you might still find this little oddity.  Below is a video of some gameplay footage.