  HUXt is an open source numerical model of the solar wind written in Python.
It is based on the solution of the 1D inviscid Burger's equation. This
reduced-physics approach produces solar wind flow simulations that closely
emulate the flow produced by 3-D magnetohydrodynamic solar wind models at a
small fraction of the computational expense. While not intended as a
replacement for 3-D MHD, the simplicity and computational efficiency of HUXt
offers several key advantages that enable experiments and the use of techniques
that would otherwise be cost prohibitive. For example, large ensembles can
easily be run with modest computing resources, which are useful for exploring
and quantifying the uncertainty in space weather predictions, as well as for
the application of some data assimilation methods.
  We present the developments in the latest version of HUXt, v4.0, and discuss
our plans for future developments and applications of the model. The three key
developments in v4.0 are: a restructuring of the models solver to enable fully
time-dependent boundary conditions, such that HUXt can in principle be
initialised with in-situ observations from any of the fleet of heliospheric
monitors; new functionality to trace streaklines through the HUXt flow
solutions, which can be used to track features such as the Heliospheric Current
Sheet; introduction of a small test-suite so that we can better ensure the
reliability and reproducibility of HUXt simulations for all users across future
versions. Other more minor developments are discussed in the article.
  Future applications of HUXt are discussed, including the development data
assimilation schemes for assimilation of both remote sensing and in-situ plasma
measures. We discuss the progress of transitioning HUXt into an operational
model at the UK's Met Office Space Weather Operations Center as part of the UK
governments SWIMMR programme.
