You can make a random number
int randomNum = Random.Range(0, 4);
And use it on a switch
with the four functions, something like this
switch(randomNum){
case 0:
switchLane1();
break;
case 1:
switchLane2();
break;
case 2:
switchLane3();
break;
case 3:
switchLane4();
break;
}
But I am sure there is a better solution for this, I see so much repeated code, maybe you can make one function with a parameter for the four cases and simply send a parameter with a direction or a number.