This book constitutes the refereed proceedings of the 4th International Joint Conference on Discrete Geometry and Mathematical Morphology, DGMM 2025, held in Groningen, The Netherlands, during November 3–6, 2025.The 37 full papers included in this book were carefully reviewed and selected from 52 submissions. They were organized in topical sections as follows: Discrete Geometry – Models, Transforms, and Visualization; Computational Aspects of Discrete Structures and Tilings; Discrete and Combinatorial Topology; Hierarchical and Graph-Based Models, Analysis and Segmentation; Algorithms; Morphological Filters, PDEs and Multivariate Morphology; Mathematical Morphology and Digital Geometry for Applications.