Example usage case for 21cmFASTv4. First, cosmological parameters are sampled, and an associated realisation of the Lagrangian matter field is generated. Dark matter halos are identified in Lagrangian space and then moved, together with the matter field, to Eulerian space using 2LPT. This is shown in the left-middle panel as a slice through a 300 Mpc overdensity field at z = 7, with the 300 largest halos in the 2 Mpc-thick slice shown as red circles. Then galaxies are assigned to DM halos by sampling parametric conditional probability densities based on well-established empirical relations such as the SHMR, SFMS, FMR, etc. Cosmic radiation fields (Lyman-alpha, Lyman-continuum, and X-ray) sourced by these galaxies are calculated using approximate radiative transfer, and the IGM is evolved accordingly. The lower panel shows slices corresponding to the IGM neutral fraction, 21cm brightness temperature, and CII surface brightness density, at the same redshift as the overdensity field and with the same halos overlaid as red circles. We can then extract statistics of these fields for comparison with multi-tracer observations. In the centre-right panel we show the ultraviolet (UV) luminosity function at z = 7, the 21cm power spectrum at the midpoint of reionisation, the evolution of the 21cm power spectrum at k = 0.1 Mpc⁻¹, and the CII x 21cm cross-power spectrum at z = 12. Under fiducial settings, 21cmFASTv4 computes all of these steps in ≲3 core hours for a single realisation, thus facilitating high-dimensional, multi-tracer, field-level Bayesian inference of cosmology and astrophysics during the EoR and CD.