The Virus Buster mode is where you'll find 4 player co-op. Multiple pills spawn at the top of the screen, and the simple grab/rotate controls allow you to move the pills freely left, right, or down (up is still locked, since this is a gravity-based puzzler at its core) to position the pieces. Each player has their own agenda, each fighting over what piece to use where.
This page contains co-op info for Dr. Mario Online on the WiiWare. All information about Dr. Mario Online was correct at the time of posting. Information is subject to change. If you see any errors please email us.