I have tried emailing WBB about this but have recieved no reply.
THis article, http://www.wannabebig.com/article.php?articleid=130 in my opinion displays a dangerous way to do the shrug, and when I saw the question 'which one shows correct form?', i thought it was a trick question as none of them do.
Apparently pictures 2 and 4 are correct, but I beg to differ.
In picture 2, the first OBVIOUS thing that guy is doing wrong, is you do NOT shrug with poor posture. Why is is head and shoulders dropping forward? His chest is NOT up out like good posture dictates. Unless you shrug with good posture, the extra force put on your spine is very damaging. Dropping your head forward when just walking or sitting can put up to 100lb of force onto your spine, imagine when you have weights in your hand. I bet his tongue is not on the roof of his mouth either. SLightly bent knees is also part of balancing load accross your body
The same issue of his head dropping forward and most of the points above are also apparent in picture 4, in picture 4 the dumbbells are also overly rotated, even though it isn't much, they should still be facing the front.
Normally I wouldn't care, but for a reputable site, exercises that are potentially dangerous (shrug because of it's relation to spinal loads) should be shown with extra care.
For a real article on shrugs, see http://t-mag.com/html/body_144shrug.html and part 2, http://t-mag.com/html/body_145shrug.html