I would guess the 4th sector is designing stuff in 3D cad software for people to 3D print, such as tinkering with mechanics of some printable machines, our customizing existing designs to each individual's preferences.
Not many people can easily learn 3D modeling (think realistically, its rather hard for average man). Moreover, cost of the 3D-printed models will be higher than production cost of non-customized bulk items on fully automated plants.