4. Option ın valuesi diyelim 4 olsun

$("#myselect").change((e)=>{
e.preventDefault();
if($("#myselect").val() == "4"){
$("<input>",{
id: "replaceselect",
class: "form-control",
keyup: ()=>{
$("#myselect").val($("#replaceselect").val());
}
}).appendTo("body");
$("#myselect").unbind();
}
})