Wich layer?

when i use this function for example …

on(rollOver) {
mc4.gotoAndPlay(2)
}

it works just fine …

ok here the mc4 mc has only one layer soo he have to play the 2nd frame in it cuz it,s the only layer …

what if the mc4 had 2 layers what should i add to make him know what layer i,m talkin about ?