Auto check checkboxes based on select field

I have working code to check checkboxes but it only works if the first checkbox is already checked. Do you guys maybe have any idea why?

HTML:

<p id="ullman_seats"><input class="choice" type="checkbox" name="option['option']" value="3000"> Ullman seats
    <select id="select_ullman">
        <option value="1">1</option>
        <option value="2">2</option>
        <option value="3">3</option>
        <option value="4">4</option>
    </select>
</p>
<p id="ullman_seats_2"><input class="choice" type="checkbox" name="option['option']" value="3000"> Ullman seats</p>
<p id="ullman_seats_3"><input class="choice" type="checkbox" name="option['option']" value="3000"> Ullman seats</p>
<p id="ullman_seats_4"><input class="choice" type="checkbox" name="option['option']" value="3000"> Ullman seats</p>

jQuery (safe because it will be implemented in WordPress)

jQuery('#select_ullman').change(function () {
        if (jQuery(this).val() == '2') {
            jQuery('#ullman_seats input').attr('checked', true);
            jQuery('#ullman_seats_2 input').attr('checked', true);
        }if (jQuery(this).val() == '3') {
            jQuery('#ullman_seats input').attr('checked', true);
            jQuery('#ullman_seats_2 input').attr('checked', true);
            jQuery('#ullman_seats_3 input').attr('checked', true);
        }if (jQuery(this).val() == '4') {
            jQuery('#ullman_seats input').attr('checked', true);
            jQuery('#ullman_seats_2 input').attr('checked', true);
            jQuery('#ullman_seats_3 input').attr('checked', true);
            jQuery('#ullman_seats_4 input').attr('checked', true);
        }
   }