Thanks to Shonumi's documentation, the 4x adapter now functions in the current dev build. You can now play 4 player GB games in BizHawk.
I tested f1-race, rc pro am, and wave race and 4 player functionality works for them.
I tested 2 player on micro machines and it works.
I tested 2 player on Yoshi's cookie and it does not work, not sure why yet. EDIT: Yoshi's Cookie is not responding to the four player adapter pings. I'm guessing it's not really compatible with it. I was testing Yoshi not Yoshi's cookie oops.
As you can see from the screen shot, it doesn't quite run at 60 fps on my laptop, I'll try to do some optimizations to get the speed up, but for now if anyone is interested feel free to play around with it and let me know of any bugs. This should still be considered WIP. EDIT: the 4 player adapter is the default connection setting in 4x mode. You don't have to press anything to get into the 4 player adapter.
This is the last major peripheral I wanted to implement personally in GBHawk. It probably doesn't have any TAS potential, I just thought it was cool. I'm not really 100% certain about the networking algorithm, I just did what made sense and followed Shonumi's documentation and it seems to work pretty well, although almost certainly not hardware accurate.
I know there are a lot of other peripherals in the gameboy domain, but I'm not super interested in them. Some of them are probably TAS worthy, If anyone feels inspired to implement more of them I can help along the way.