The players are the four colored objects; avoid red or orange enemies and seek out circular powerups. Each player starts with 3 lives.
Player 1 uses arrows keys and pageup (you may want to map that to a different key if your keyboard is different from mine).
Player 2 uses wasd and q.
Player 3 uses tfgh and r.
Player 4 uses ijkl and u.
It should be easy to add more players if you want; if you want to play with less than four people, it should also be easy to disable the rest or just let them die.
The green powerup is an extra life.
The yellow powerup is speed.
The blue powerup is a time stop (does not freeze players or powerup spawning).
The grey powerup gives you three bullets that will fire in the direction you're moving. Use your fifth key to shoot.
Have fun!