Four joysticks on Atari ST
I could not find a prebuilt 4 joystick adapter (also known as “multitap adapter”) to Atari ST. Being lazy, I picked the second easiest option and converted an Amiga adapter to Atari ST one. It was pretty easy even for me. Just rewire one pin, and you are good to go.
If you want to build it all yourself, refer to Atari Forum Wiki or find the details on ST Action, Issue 5 (pages 16-17) or ST Format, Issue 18 (page 184).
The list of games supporting 4 joysticks is not huge, but just having Gauntlet II and Kick Off 2 should be worth it. All we need now is MORE PLAYERS!