Would it technically be possible to have the USB-2-CD-i adapter to support four controllers and create a four player homebrew CD-i game?

It will probably be a wild idea forever, but since the recent developments of Nobelia on CD-i (homebrew game) and the USB2CDi adapter (To connect USB controllers to your CD-i player) come from one and the same person, PLUS history has shown a similar effort, it is not that impossible. 

When we were discussing the possibilities of the CD-i splitter (and multiplayer games on CD-i); Playing locally Rise of the Robots with two players is a nice experience on CD-i. Jeffrey mentioned his USB2CDi Adapter also supports as a splitter and you can connect two controllers to it, which is a neat technology. Back in the day, the guys behind Creative Media (who developed CD-i games like Christmas Country, Whack a Bubble and Tetsuo Gaiden) were experimenting with the Family Box Plug, which was a four-player adapter, so you could play with four players simultaneousley on CD-i. Next to that, they developed games like Taco's Toyroom Troopers, which was a tank game with four players and you had to shoot each other. I wondered if the USB2CDi Adapter would in principle be suitable to serve like a four player adapter as well. If Jeffrey can make Nobelia on CD-i, a four player multiplayer game like Taco's Toyroom Troopers would, in theory at least, be possible. Even when it is limited to two controllers only, you could share the button layout and for example let the tank accelerate automatically and steer with two buttons, just like what Codemasters did in Micro Machines on CD-i.