Time.
The fourth dimension in 4D printing refers to materials that are able to change and mutate when exposed to water, temperature changes and/or air to self assemble. 4D object formats have API’s (Application Programmers Interfaces) that enable designers to define the characteristics of the materials they are made from, which are then printed using sophisticated chemical calibrations to enable specific attributes and functionality.
From www.zdnet.com
Nanotech has been on its way for so long, some people have stopped believing in its potential. It still doesn’t seem like a real thing.