Rapid prototyping makes additive manufacturing (or 3D printing) useful in critical application domains such as aerospace, automotive, and medical. The rapid expansion of these applications should prompt the examination of the underlying security of 3D printed objects. In this paper, we present Mystique, a novel class of stealthy attacks on printed objects that leverage the fourth dimension of emerging 4D printing technology to introduce embedded logic bombs through manufacturing process manipulation. Mystique enables visually benign objects to behave maliciously upon the activation of the logic bomb during operation. It leverages the manufacturing process to embed a physical logic bomb that can be triggered with specific stimuli to change the physical and mechanical properties of the printed objects. These changes in properties can potentially cause catastrophic operational failures when the objects are used in critical applications such as drones, prosthesis, or medical applications.