The ReMotion Knee lets people in the developing world share some of our advanced prosthetic technology, but at a price you can afford if you make $4 a day. After he lost his leg in a traffic accident, Sonu Kumar, a 20-year-old electrician in Jaipur, India, was one of the first people to test an early prototype for a low-cost prosthetic that helped him comfortably walk again—and become so mobile he was eventually able to…