Imagine flat-packed furniture that can assemble itself, or a water pipe that can repair itself if it breaks or bursts. Items that are too large for a 3D printer can be printed using a compressed design and then self-assemble on its own.
Just as importantly, many 4D-printed objects are being designed to return to their shape once the stimuli are removed. Imagine a building that repositions itself to brace for an earthquake, then shrinks back to normal afterward.

https://store.hp.com/app/tech-takes/4d-printing-already?utm_content=S2R5C1_Mod3&jumpid=em_com_nc_ns&aoid=190122740&utm_medium=em&utm_source=sf&rid=FEB729CE1C71A114984CB5613FD75719&test=&jobid=1901227&emailid=100185