In Sanhedrin (77b) the gemara discusses a case of "Wall Ball" throwing a ball at a wall, the ball bouces back and if the other player catches the ball, he tries to peg the original thrower (Rashi). The case the gemara brings down is one where the original thrower threw the ball and it ended up killing the second player. (Note although it may be able to implied, the gemara doesn't out right say the ball bounced off the wall and killed the second player). The gemara goes on to say, within 4 amot your פטור past 4 and your חייב (Rashi says 4 amot from the wall, note: the gemara doesn't specify what your חייב and פטור)
I'm curious if anyone can elaborate on what exactly wall ball was (what kinda ball..etc) anything that could help clear up this case. Although the gemara mentions later on that the reason 4 amot is significant is because people try to get the ball past 4 amot, in a case where one is trying to kill his opponent, he will purposely aim for him off the wall and so why then is 4 amot significant? He should be חייב regardless