With a new custom-designed 4-player joystick adapter, QuadGames is the first multi-cart of 4-player joystick-based Atari games!
It is currently a work-in-progress, but the current games to be included are:

• Quad-Joust
• Quad-Tank

Development of the game is in-progress as of late 2017. A single prototype of the joystick adapter has been made, and current plans are to investigate manufactuing for the adapter, so that others can purchase and play these games.

Currently, I also have a forked version of the Stella emulator that includes (buggy) support for this adapter. Contact me if you are interested in testing or trying it out.