No animal on Earth has four knees, only two. All animal skeletons consist of essentially the same bones and joints, just expressed in different shapes or forms.
All creatures that exist with four limbs, comprising the legs and arms, fall in the category of tetrapods. This word comes from Greece and means "four legs." The animals in this group all share the same basic skeletal structure, which consists of two arms, two legs, a torso and a head. Since all animals belong to this group, all animals have only two "real" legs and two "real" arms. So, although evolution may have shaped some species' arms into shapes that appear to be legs, they are still essentially arms.
Take a horse for instance. The two front legs are actually arms, just shaped that way over millions of years of evolutionary pressure. Because of this, there are no physiological knees within the front two legs of a horse. They are actually elbows that have been shaped to serve a similar purpose to the kneecap on the hind legs. In fact, horses have specialized their arm shape so drastically over the years that their hooves are actually nothing but their middle fingers.
It should be pointed out that not every tetrapod is equipped with four limbs in modern times. Whales have partially lost some of their limbs, with fragments still being visible in skeletons. Additionally, snakes have completely lost all four of theirs.