I dunno, they don't really show a picture of Kaidou but i know he one of these two strangers : http://onepiece.wikia.com/wiki/File:Yonkou.jpg
If Kaidou is the guy that has bushy hair then I guess the 4th Yonkou is a Merman.
If Kaidou is the guy on the bottem, then the bushy hair guy is probably a zombie or giant.