  The Rivet toolkit is the primary mechanism for phenomenological preservation
of collider-physics measurements, containing both a computational core and API
for analysis implementation, and a large collection of more than a thousand
preserved analyses. In this note we summarise the main changes in the new Rivet
4 major release series. These include a major generalisation and more
semantically coherent model for histograms and related data objects, a thorough
clean-up of inelegant and legacy observable-computation tools, and new systems
for extended analysis-data, incorporation of preserved machine-learning models,
and serialization for high-performance computing applications. Where these
changes introduce backward-incompatible interface changes, existing analyses
have been updated and indications are given on how to update new analysis
routines and workflows.
