Dynamic Substructures, Volume 4*: Proceedings of the 39th IMAC, A Conference and Exposition on Structural Dynamics, 2021,* the fourth volume of nine from the Conference brings together contributions to this important area of research and engineering. The collection presents early findings and case studies on fundamental and applied aspects of the Dynamics of Coupled Structures, including papers on: Methods for Dynamic Substructures Applications for Dynamic Substructures Interfaces & Substructuring Frequency Based Substructuring Transfer Path Analysis