The 4th EOS Topical Meeting on Blue Photonics - Optics in the Sea (Blue Photonics 4), will be held at the Institute of Marine Sciences (ICM, CSIC), Barcelona, Spain, and will provide a major international forum for the discussion of marine optics, underwater imaging systems and its applications. Its aims are to review recent progress in the field, to promote the formation of a community of scientists and engineers working in related areas, and to stimulate the interdisciplinary collaboration required to meet the rapidly developing challenges of the future. Topics will include: Environmental Monitoring, New low-cost technologies, Ocean Color, Radiative Transfer and Modelling, Marine Bio-optics and Remote Sensing, Fluorometry, AOP - IOP 2-D and 3-D, Underwater Imaging, Innovative Sub-sea Optical Techniques and Instrumentation.