Step away from Sonic 4 for a moment -- you might want to try this out. Sonic Fan Remix is a custom visual update for Sonic the Hedgehog 2 and the effort looks very polished so far:
The visuals may be hit or miss depending on who you ask but what matters most is that Sonic's controls are dead-on, even when playing on a keyboard. There are also some nice details everywhere such as the Robotnik mech in the background and the occasional burning palm tree.
If you're going to try out the demo in the source link, there's a couple of things to take note: Sonic Fan Remix requires a rather powerful system to run but it'll still look great even when set at the lowest graphics settings. When you're ready to quit the game, you'll have to use the ol' Alt+F4 to exit.
Update: The download has been taken down due to a copyright notice, possibly by Sega. We'll try to get a comment from them if possible.