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.
It's actually not that difficult any more. When I first got into it, 3D modeling was done with code, such as
Sphere (2, 2, 1.5, 4, "yellow")
Cube(6, 2, 1.5, 2, "blue")
(XYZ coordinates, diametes, and defined color or texture parameter)
which would then have to be rendered, slowly, one frame at a time, and once you see where the error is, go back and adjust the code, and then render again to see if it fixed it. Now you can use a mouse to move things around on your screen in real time. We're also getting to the point where 3D manipulation tools are becoming cheap, allowing you to reach toward your screen, "grab" something, and move it back and forth, instead of just up/down/left/right, like a mouse lets you. As 3D printing and 3D design becomes more prevalent, this will be much more common place, and instead of workers standing in front of work tables, physically modifying things, you'll have workers standing in front of computer terminals, building and modifying things on computer. There is already a pretty good first-look example of this type of work in SecondLife, where people earn a living by designing objects using 3D tools (clothing, furniture, random junk, and even houses), and sell them in virtual stores. Dspite materials not costing anything, creativity still pays there.
As for bulk mass production, those same 3D tools can be used to 3D print molds for mass production, too.