copy function

Hi
Please, can anyone help me solve this question?
I created this js acrobat function

var s1=this.getField("RESIDENT IN").value;
var s2=this.getField("RESIDENT IN_2").value;
var a1=this.getField("VIA").value;
var a2=this.getField("VIA_2").value
var b1=this.getField("N").value;
var b2=this.getField("N_2").value;
var residence1=s1+a1+b1;
var residence2=s2+a2+b2;
if(residence2==residence1)event.value=this.getField("4").value;

I inserted this function in the calculation script of field 4_2

I would like that when field 4 is empty, you can freely write any text in field 4_2