With 3D printing, three-dimensional objects can be created. 4D printing adds the fourth dimension of time savings. While 3D printing produces complex objects from several individual objects that then have to be assembled, 4D printing allows complete complex objects to be printed. The time saved for assembly is interpreted as the fourth dimension.
In terms of printing processes, 4D printing corresponds to additive 3D printing processes. Thus, 4D printing is about the time savings that result from the fact that 4D printers can print more complex 3D objects, resulting in less assembly time.