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We present the new public version of the KETJU supermassive black hole (SMBH)dynamics module, as implemented into GADGET-4. KETJU adds a small region aroundeach SMBH where the dynamics of the SMBHs and stellar particles are integratedusing an algorithmically regularised integrator instead of the leapfrogintegrator with gravitational softening used by GADGET-4. This enablesmodelling SMBHs as point particles even during close interactions with stellarparticles or other SMBHs, effectively removing the spatial resolutionlimitation caused by gravitational softening. KETJU also includespost-Newtonian corrections, which allows following the dynamics of SMBHbinaries to sub-parsec scales and down to tens of Schwarzschild radii. Systemswith multiple SMBHs are also supported, with the code also including theleading non-linear cross terms that appear in the post-Newtonian equations forsuch systems. We present tests of the code showing that it correctly captures,at sufficient mass resolution, the sinking driven by dynamical friction andbinary hardening driven by stellar scattering. We also present an exampleapplication demonstrating how the code can be applied to study the dynamics ofSMBHs in mergers of multiple galaxies and the effect they have on theproperties of the surrounding galaxy. We expect that the presented KETJU SMBHdynamics module can also be straightforwardly incorporated into other codessimilar to GADGET-4, which would allow coupling small-scale SMBH dynamics tothe rich variety of galactic physics models that exist in the literature.

KETJU -- resolving small-scale supermassive black hole dynamics in GADGET-4