word limit in a comments/status update box

I'm looking for someone who can create a comments box that limits the number of words that can be typed in it.  This would basically measure spaces, ignore double spacing, and would allow for commas, full stops, exclamation marks and other punctuation.  I plan to run a caption competition where contestants enter a photo with a 4 word limit for the caption.  Any ideas?

Thanks in advance,

Tom.