Thinking that when my project will be complete, I will have to insert two other characters (and it will be hard to do it),
I trained by making a nice example that could be useful as a basis for creating four-player arcade games simultaneously.
In this tuotorial there is also an example of how to limit the directions of the shot only to the right and left (in some genres of games, when the directions of the shot are limited, they are more playable, so even you have to use only one button).
But warning, this is only a basis, because someone here could expect a full game, and could be disappointed!
Download the example here