From 2016 to 2021 the research group HistStadt4D investigated and developed methods and technologies to transfer extensive repositories of historical photographs and their contextual information into a 3D spatial model, with an additional temporal component. The aim was to make content accessible to researchers and the public, via a 4D browser and location-dependent augmented reality representation. Against this background, in this article we present the achievements of the project, lessons learned, and current state of 4D urban history research and discovery based on historical photographs.