AN profiles four young designers who want to make a difference. RISD-trained Jessica Carnevale has parlayed stints at Martha Stewart Living and Marcel Wanders Studio into an exuberant, fashion-tinged collection. Brooklyn-based Karl Zahn is making the quantum leap from cottage industry to a future with large manufacturers. Tokyo-born Jonah Takagi is launching an American craft-based collection for Matter at ICFF. And Brooklyn’s Kristen Wentrcek applies an entrepreneurial spirit to bringing space-starved New Yorkers affordable, inventively designed furniture.