This is the fourth paper in our Light-Emitting Diodes series. The series aims to create a systematic library of LED-based materials and to provide readers with the description of experiments and the pedagogical treatment that would help their students construct, test, and apply physics concepts and mathematical relations. The first paper provided an overview of possible uses of LEDs in physics courses. The second paper discussed how one could help students learn the foundational aspects of LED physics through a scaffolded inquiry approach, specifically the ISLE cycle. The third paper showed how the physics inherent in the functioning of LEDs could help students deepen their understanding of sources of electric power and the temperature dependence of resistivity, and explore the phenomenon of fluorescence also using the ISLE cycle. The goal of this fourth paper is to use LEDs as black boxes that allow students to study certain properties of a system of interest, specifically mechanical, electric, electromagnetic, and light properties. The term "black box" means that we use a device without knowing the mechanism behind its operation.