Planetary caves have been identified on the Moon and on Mars, and are likely to occur across the Solar System. They present a new frontier for planetary science, subsurface astrobiology, geology and human exploration. The fourth in a series of scientific meetings focusing on the science and exploration of planetary caves brought together 55 terrestrial and planetary scientists, robotics and instrumentation engineers, and students (16 including 1 undergraduate). Conference participants discussed the state of the art of relevant sciences and current engineering capabilities as applied to planetary cave exploration and research. Specifically, they considered cave formation mechanisms, preserved geological records, cave micro-climate and astrobiological potential, engineering challenges of subsurface exploration, and potential robotic mission concepts to explore the subsurface of other worlds, especially the Moon and Mars.