  An analysis of high-resolution ($R\sim48\,000$) optical spectrum of hot
(B1Ibe) post-AGB star LS 4331 (IRAS 17381-1616) is presented. The detailed
identification of the observed absorption and emission features in the
wavelength range 3700-9200 \r{A} is carried out for the first time. From
non-LTE analysis of absorption lines the atmospheric parameters and chemical
composition of the star are derived. We estimate $T_{\rm eff}=20~900\pm500$ K,
$\log g=2.57\pm0.08$, $V_r=-51.7\pm0.8$ km s$^{-1}$, $\xi_{\rm t}=24\pm4$ km
s$^{-1}$ and $v \sin i=30\pm5$ km s$^{-1}$. A abundance analysis for C, N, O,
Mg, Al, S and Si reveals that the N and O abundance is close to solar while
metal underabundances relative to the solar value (i.e. [Mg/H] = -1.04 dex,
[Al/H] = -1.20 dex, [Si/H] = -0.46 dex) are found. We conclude that LS 4331 is
a high galactic latitude metal-poor and carbon deficient hot post-AGB star. The
underabundance of carbon ([C/H]= -0.64 dex) is similar to that found in other
hot post-AGB stars and indicates that the star's AGB phase of evolution was
terminated before the third dredge up. From the nebular emission lines the
plasma diagnostics are derived. The presence of nebular emission lines in the
spectrum of LS 4331 indicates that the photoionization of circumstellar
envelope has already started. The nebular parameters and expansion velocity of
the nebula is derived. Using the Gaia DR3 distance the absolute luminosity of
the star is derived and the star's position on the post-AGB evolutionary tracks
suggests that its initial main sequence mass is about 1.2$M_{\odot}$. It is
also reported that fast irregular brightness variations with an amplitude of up
to 0.3 mag in $V$ band have been found in the star, typical of hot post-AGB
objects.
