Multistep form - problem with field filled by jquery

Multistep form - problem with field filled by jquery

Hi Everyone,
I have problem with script who filled input field when one value is less then 4. When the value is less then 4 the script filling field but the value is not selected, so when I'm going to next step I have no value in this field. When the value is greater then 4 everything is ok (the value is also filling by jquery).

This is my code: 
  1. function compute() {
  2.       if( parseInt($("#finish_day").val()) < 4 ) {  
                $('#return_car').children('#return_car option[value=' + $('#get_car').val() + ']').attr('selected', true).siblings().attr('disabled', true);

                if ($('#return_car').val()) $('#return_car').change();
            }
            else {
                $('#get_car > option, #return_car > option').prop('disabled', false);
                $('#three_day').hide(2000, function () {
                    $(this).remove();
                });
            }

        }











  3. $('select#get_car').change(compute);
    $('input#finish_day').change(compute);

Where is the problem? Thanks for any help