Beautiful tone that really emphasizes the various textures in the photo. My one nit-pic would be a slightly deeper depth of field to get the 2 back legs in focus, since your title references all four legs. But I'm not holding it against you, much! 9
I like this shot because of the uniquness I think and the tones used. Its nicely framed. The left rear leg somehow seems to dominate the shot a bit though. Hope you use a telephoto, dangerous to get too close behine a horse ;-)