Use random

define attack_punch,0;
define attack_kick,1;
define attack_highpunch,2;
define attack_lowkick,3;

var ran = int(random(4));

if(ran == attack_punch) {punch();}
if(ran == attack_kick) {kick();}
if(ran == attack_highpunch) {highpunch();}
if(ran == attack_lowkick) {lowkick();}