It would be good to discuss some concrete positions before the game starts, to make sure that we haven’t missed some big problems with the rules. Let’s start with a relatively basic one:
The current stone counts are:
21
17
16
12
Blue, red and yellow have enough eyes to avoid capture unless they decide to fill in their own eyes. Black could potentially be captured, but it is not in yellow’s interest to help.
A possible outcome is that everyone waits around until black is eliminated (we should probably have an option to “fast forward” a few turns, without anyone making moves, if everyone agrees). Red, blue and yellow can then safely fill in another one of their own eyes, leaving everyone with 3 eyes exactly. Blue still has fewer stones than red, so their last hope is to fill another eye. This gives yellow the opportunity to kill blue together with red, but yellow doesn’t have to do that to win, so let’s assume that yellow refuses. Then red has to decide between sharing second place with blue, or follow along and also fill in another eye, which will give yellow the opportunity to kill red together with blue (but again yellow has no intrinsic motivation to do so). If yellow doesn’t capture anybody, the game ends with this ranking:
Yellow > Red > Blue > Black
Now go back to the beginning, and consider what happens if black fills in one of their own eyes, allowing blue and red to capture. What happens afterwards is a bit complicated, but it looks like red can easily secure enough area to win. Let’s assume that blue gets exactly 5 more stones, putting them on equal footing with yellow. We are then in a situation similar to earlier, where blue or yellow might fill in their eyes, allowing red the opportunity to capture. If red doesn’t capture anybody, the game ends with the ranking:
Red > Yellow = Blue > Black
With the cooperation of red, either yellow or blue could beat out the other in the final ranking (but capturing looks risky for red, so I think the above is the most likely outcome).
Notice that there are lots of cases where it’s possible to impact the final ranking of other players in a very direct way. It’s not only the players with strong board positions that have this control: In the above example, yellow looks the strongest and black the weakest. But black can choose to deny yellow the victory.
In conclusion, being on good terms with other players is extremely important.