  We provide one of the most comprehensive metallicity studies at z $\sim$ 4 by
analyzing the UV/optical HST photometry, and rest-frame VLT-FORS2 ultraviolet
and VLT-XSHOOTER optical spectra of J0332-3557, a gravitationally lensed galaxy
magnified by a factor of 20. With a 5$\sigma$ detection of the auroral O
III]1666 line, we are able to derive a direct gas metallicity estimate for our
target. We find Z_gas = 12 + log(O/H) = 8.26 $\pm$ 0.06, which is compatible
with an increasing of both the gas fraction and the outflow metal loading
factor from z $\sim$ 0 to z $\sim$ 4. J0332-3557 is the most metal-rich
individual galaxy at z $\sim$ 4 for which the C/O ratio has been measured. We
derive a low log(C/O) = - 1.02 $\pm$ 0.2, which suggests that J0332-3557 is in
the early stages of ISM carbon enrichment driven mostly by massive stars. The
low C/O abundance also indicates that J0332-3557 is characterized by a low star
formation efficiency, higher yields of oxygen, and longer burst duration. We
find that EW(C III])1906,9 is as low as $\sim$ 3 \AA. The main drivers of the
low EW(C III])1906,9 are the higher gas metallicity and the low C/O abundance.
J0332-3557 is characterized by one diffuse and two more compact regions $\sim$
1 kpc in size. We find that the carbon emission mostly originates in the
compact knots. Our study on J0332-3557 serves as an anchor for studies
investigating the evolution of metallicity and C/O abundance across different
redshifts.
