Simple.
Two combos, one has names, one has numbers


In the change event (javascript) for DropDownList1 put:
if ($.boundItem(xfa.event.newText) == "Bill") {
this.resolveNode("DropDownList2").rawValue = "0000";
}
else if ($.boundItem(xfa.event.newText) == "John") {
this.resolveNode("DropDownList2").rawValue = "1111";
}
else if ($.boundItem(xfa.event.newText) == "Roberta") {
this.resolveNode("DropDownList2").rawValue = "2222";
}
NOTE: Unless you make your second combo readonly, you will be able to select a different number regardless of what is selected in the first.