4D-printed mechanical metamaterials: responsive architectures for programmable and adaptive functions

4D-printed mechanical metamaterials combine stimuli-responsive materials with architected mechanical designs, enabling printed structures to change shape, stiffness, deformation mode, or function after fabrication. This review discusses these systems as responsive architectures in which material response and geometry-driven mechanics are jointly designed. We summarize representative material platforms, external stimuli, and additive manufacturing routes, including shape-memory polymers, hydrogels, liquid crystal elastomers, conductive composites, field-responsive systems, and multimaterial hybrids. We then discuss structural design based on geometry, stiffness contrast, gradients, multistability, and topological transformation, along with the modeling and inverse-design methods used to support these designs. Representative applications are reviewed in programmable reconfiguration, wave and vibration control, recoverable energy absorption, sensing-integrated systems, intelligent regulation, and biomedical devices. Finally, remaining challenges are discussed, including slow actuation, fatigue resistance, multimaterial interface stability, quantitative programming, scalable fabrication, and standardized evaluation. These issues will shape the development of reliable adaptive mechanical systems.