  Dusty star-forming galaxies emit most of their light at far-IR to mm
wavelengths as their star formation is highly obscured. Far-IR and mm
observations have revealed their dust, neutral and molecular gas properties.
The sensitivity of JWST at rest-frame optical and near-infrared wavelengths now
allows the study of the stellar and ionized gas content. We investigate the
spatially resolved distribution and kinematics of the ionized gas in GN20, a
dusty star forming galaxy at $z$=4.0548. We present deep MIRI/MRS integral
field spectroscopy of the near-infrared rest-frame emission of GN20. We detect
spatially resolved \paa, out to a radius of 6 kpc, distributed in a clumpy
morphology. The star formation rate derived from \paa\ (144 $\pm$ 9
\msunperyear) is only 7.7 $\pm 0.5 $\% of the infrared star formation rate
(1860 $\pm$ 90 \msunperyear). We attribute this to very high extinction (A$_V$
= 17.2 $\pm$ 0.4 mag, or A$_{V,mixed}$ = 44 $\pm$ 3 mag), especially in the
nucleus of GN20, where only faint \paa\ is detected, suggesting a deeply buried
starburst. We identify four, spatially unresolved, clumps in the \paa\
emission. Based on the double peaked \paa\ profile we find that each clump
consist of at least two sub-clumps. We find mass upper limits consistent with
them being formed in a gravitationally unstable gaseous disk. The UV bright
region of GN20 does not have any detected \paa\ emission, suggesting an age of
more than 10 Myrs for this region of the galaxy. From the rotation profile of
\paa\ we conclude that the gas kinematics are rotationally dominated and the
$v_{rot}/\sigma_{m} = 3.8 \pm 1.4$ is similar to low-redshift LIRGs. We
speculate that the clumps seen in GN20 could contribute to building up the
inner disk and bulge of GN20.
