Mechanical metamaterials owe their extraordinary properties and functionalities to their micro-/nanoscale design of which shape, including both geometry and topology, is perhaps the most important aspect. 4D printing enables programmed, predictable, and precise change in the shape of mechanical metamaterials to achieve multi-functionality, adaptive properties, and the other types of desired behaviors that cannot be achieved using simple 3D printing. This paper presents an overview of 4D printing as applied to mechanical metamaterials. It starts by presenting a systematic definition of what 4D printing is and what shape aspects (e.g., geometry, topology) are relevant for the 4D printing of mechanical metamaterials. Instead of focusing on different printing processes and materials, the paper addresses the most fundamental aspects of the shapeshifting behaviors required for transforming a flat construct to a target 3D shape (i.e., 2D to 3D shapeshifting) or transforming a 3D shape to another 3D shape (i.e., 3D to 3D shapeshifting). In either case, we will discuss the rigid-body shape morphing (e.g., rigid origami) as well as deformable-body shapeshifting. The paper concludes with a discussion of the major challenges ahead of us for applying 4D printing to mechanical metamaterials and suggests several areas for future research.