  We present rest-frame optical data of the z~4 sub-millimeter galaxy GN20
obtained with JWST/NIRSpec in integral field spectroscopy (IFS) mode. The
H$\alpha$ emission is asymmetric and clumpy and extends over a projected
distance of more than 15 kpc. To first order, the large-scale ionised gas
kinematics are consistent with a turbulent ($\sigma\sim90$ km/s), rotating disc
($v_{\rm rot}\sim500$ km/s), congruent with previous studies of its molecular
and ionised gas kinematics. However, we also find clear evidence for
non-circular motions in the H$\alpha$ kinematics. We discuss their possible
connection with various scenarios, such as external perturbations, accretion or
radial flows. In the centre of GN20, we find broad line emission (FWHM
$\sim1000-2000$ km/s) in the H$\alpha$+[N II] complex, suggestive of fast,
AGN-driven winds or, alternatively, of the broad-line region of an active black
hole. Elevated values of [N II]$\lambda6583$/H$\alpha>0.4$ and EW(H$\alpha)>6$
\r{A}, throughout large parts of GN20 suggest that feedback from the active
black hole is able to photo-ionise the interstellar medium. Our data
corroborates that GN20 offers a unique opportunity to observe key processes in
the evolution of the most massive present-day galaxies acting in concert, over
12 billion years ago.
