  Galactic outflows driven by star formation or active galactic nuclei are
typically formed by multi-phase gas whose temperature spans over 4 orders of
magnitude. Probing the different outflow components requires multi-wavelength
observations and long exposure times, especially in the distant Universe. So
far, most of the high-z studies have focused on a single gas phase, but this
kind of analysis may potentially miss a non-negligible fraction of the total
outflowing gas content. In this work, we analyze the spatially resolved
rest-frame UV and optical emission from HZ4, the highest redshift main sequence
star-forming galaxy having a detected [C II] outflow, which traces the neutral
gas component. Our goal is to study the ionized interstellar medium in the
galaxy and the properties of the ionized outflow as traced by the [O
III]$\lambda$5007\r{A} and H$\alpha$ emission lines. We exploit JWST/NIRSpec
observations in the integral field spectroscopy mode to investigate the galaxy
properties by making use of the brightest rest-frame optical emission lines.
Their high spectral and spatial resolution allows us to trace the ionized
outflow from broad line wings and spatially resolve it. We also re-analyze the
[C II] ALMA data to compare the neutral atomic and ionized outflow
morphologies, masses, and energetics. We find that the system consists of a
galaxy merger, instead of a rotating disk as originally inferred from
low-resolution [C II] observations, and hosts an extended ionized outflow. The
ionized outflow is being launched from a region hosting an intense burst of
star formation and extends over 4 kpc from the launch site. The neutral and
ionized outflows are almost co-spatial, but the mass loading factor in the
ionized gas phase is two orders of magnitude smaller than in the neutral phase,
as found for other lower redshift multi-phase outflows.
