  For over 15 years, the mlpack machine learning library has served as a "swiss
army knife" for C++-based machine learning. Its efficient implementations of
common and cutting-edge machine learning algorithms have been used in a wide
variety of scientific and industrial applications. This paper overviews mlpack
4, a significant upgrade over its predecessor. The library has been
significantly refactored and redesigned to facilitate an easier
prototyping-to-deployment pipeline, including bindings to other languages
(Python, Julia, R, Go, and the command line) that allow prototyping to be
seamlessly performed in environments other than C++. mlpack is open-source
software, distributed under the permissive 3-clause BSD license; it can be
obtained at https://mlpack.org
