You don't know what the next sector will be, so there is no way you can definitively say that there is no 4th sector to move into.
It is clear that only one candidate for the 4th sector is fundamental science (nuclear fusion, planet colonization, relativistic-speed travel etc). But these mega-projects could take centuries to be completed and have uncertain ±ROI at all, therefore unlikely they will appear in countries with free-market capitalism.
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.