The Sonic 4 ROM was dumped by someone whose name is an anagram of STD.

Sonic 4 is based on a Speedy Gonzales game.

The sprites are quite good.

Once the first level is completed, the game freezes.

Other levels can be accessed by savestate hacking.

Of course, these levels could be accessed in the original Speedy Gonzales game.

Another level.

Here's the cart.

And another.