Waveguide Quantum ElectroDynamics (QED) is an emerging area of quantum optics that investigates the coherent coupling between one or more quantum emitters and a photonic waveguide or, more generally, an engineered photonic bath. The workshop aims at bringing together major experts working on this topic. This is the fourth edition of WQED workshops series. The first three editions were held in 2018, 2021 and 2023.