Level 13 is all the same like level 12 with just one difference. This time the image file we need to look for is different. Its images/lvl13.gif. If you open this image file you will find that Level 13 is in fact an image. And if you enlarge this image and look at it carefully you will find 4.xml written in the last l of level.
You can try opening this 4.xml but it won’t do any good, its just because its 4xml.php.