4 Way Play
From Sega Retro
The 4 Way Play is an accessory created by Electronic Arts for the Sega Mega Drive. It allows for up to four players to play together in certain games. Largely, mostly games designed by EA itself supported it. When first released, Sega's official Team Player multi-player adaptor was not compatible with 4 Way Play games. Later releases fixed this incompatibility issue.
The device itself plugs directly into the system and has no wires sticking out and attaches to the front of the system like an extension. To attach like this it plugs into both controller ports. Due to the way it is plugged into the system it is incompatible with devices such as the Sega Multi-Mega and X'eye.