The physics of wave propagation in disordered, structured or nonlinear media is at the heart of various research fields covering disordered quantum gases, electromagnetic waves in scattering media or wave topology in fundamental physics, wave-front shaping or nonlinear-imaging in applied physics. This 4th annual meeting of the GdR Complexe aims at gathering physicists to discuss these topics and their connections, as well as to identify emerging questions around the propagation of waves in complex systems in a broad sense.