Can't get code to work to multiple check boxes. Forms Javascript.

I want to be able to  check checkbox 1, then checkbox 2. If both of those boxes are checked I want checkbox 4 to be automatically checked. This is what I have tried but it does not work. Any help would be great. Thanks. 

var checkbox1 = this.getField("Checkbox1").value;
var checkbox2 = this.getField("Checkbox2").value;

if (checkbox1 === "Yes" && checkbox2 === "Yes") {
this.getField("Checkbox4").value = "Yes";
} else {
this.getField("Checkbox4").value = "Off";
}