  Direct imaging studies have mainly used low-resolution spectroscopy
($R\sim20-100$) to study the atmospheres of giant exoplanets and brown dwarf
companions, but the presence of clouds has often led to degeneracies in the
retrieved atmospheric abundances (e.g. C/O, metallicity). This precludes clear
insights into the formation mechanisms of these companions. The Keck Planet
Imager and Characterizer (KPIC) uses adaptive optics and single-mode fibers to
transport light into NIRSPEC ($R\sim35,000$ in $K$ band), and aims to address
these challenges with high-resolution spectroscopy. Using an atmospheric
retrieval framework based on petitRADTRANS, we analyze KPIC high-resolution
spectrum ($2.29-2.49~\mu$m) and archival low-resolution spectrum ($1-2.2~\mu$m)
of the benchmark brown dwarf HD 4747 B ($m=67.2\pm1.8~M_{\rm{Jup}}$,
$a=10.0\pm0.2$ au, $T_{\rm eff}\approx1400$ K). We find that our measured C/O
and metallicity for the companion from the KPIC high-resolution spectrum agree
with that of its host star within $1-2\sigma$. The retrieved parameters from
the $K$ band high-resolution spectrum are also independent of our choice of
cloud model. In contrast, the retrieved parameters from the low-resolution
spectrum are highly sensitive to our chosen cloud model. Finally, we detect CO,
H$_2$O, and CH$_4$ (volume mixing ratio of log(CH$_4$)=$-4.82\pm0.23$) in this
L/T transition companion with the KPIC data. The relative molecular abundances
allow us to constrain the degree of chemical disequilibrium in the atmosphere
of HD 4747 B, and infer a vertical diffusion coefficient that is at the upper
limit predicted from mixing length theory.
