  We report observations of a powerful ionized gas outflow in a z = 4.1
luminous ($ L_{1.4GHz} \sim 10^{28.3} \ W \ Hz^{-1}$) radio galaxy TNJ1338-1942
hosting an obscured quasar using the Near Infrared Spectrograph (NIRSpec) on
board JWST. We spatially resolve a large-scale (~15 kpc) outflow and measure
resolved outflow rates. The outflowing gas shows velocities exceeding 900 $ km
\ s^{-1}$ and broad line profiles with line widths exceeding 1200 $ km \
s^{-1}$ located at ~10 kpc projected distance from the central nucleus. The
outflowing nebula spatially overlaps with the brightest radio lobe, indicating
that the powerful radio jets are responsible for the extraordinary kinematics
exhibited by the ionized gas. The ionized gas is possibly ionized by the
central obscured quasar with a contribution from shocks. The spatially resolved
mass outflow rate shows that the region with the broadest line profiles
exhibits the strongest outflow rates, with an integrated mass outflow rate of
~500 $ M_{\odot} \ yr^{-1}$. Our hypothesis is that an over-pressured shocked
jet fluid expands laterally to create an expanding ellipsoidal "cocoon" that
causes the surrounding gas to accelerate outwards. The total kinetic energy
injected by the radio jet is about 3 orders of magnitude larger than the total
kinetic energy measured in the outflowing ionized gas. This implies that
kinetic energy must be transferred inefficiently from the jets to the gas. The
bulk of the deposited energy possibly lies in the form of hot (~$ 10^7$ K)
X-ray-emitting gas.
