ASPECT is a parallel, extendible finite element code to simulate thermal convection in both 2D and 3D models. This release includes the following changes: Complete overhaul of the particle architecture. Modularization of the assembly process. Large improvements to serial and parallel performance of particle handling. Support for traction boundary conditions. Support for time-dependent mesh refinement criteria. Flexible support for modifying the underlying FEM variables for the PDE. Optional DG support for temperature and compositional fields. More robust parallel file I/O. Support for tangential mesh velocity boundaries in free surface computations. Support for anisotropic viscosity. Various fixes to free surface computations: checkpointing, better stabilization, crash fixes. New Chunk Geometry model. New material property averaging options. Complete rewrite of the heating model infrastructure. Several new cookbooks. Several new postprocessors. Support for signals in various locations that allows plugins to inspect/manipulate things inside the core application. Several new mesh refinement plugins. Improved spherical interpolation of data used in the GPlates plugin. Many other fixes and small improvements. A complete list of changes can be found at https://aspect.dealii.org/doc/doxygen/changes_between_1_83_and_1_84_80.html and the release is also available from https://github.com/geodynamics/aspect/releases/tag/v1.4.0