  Spectra of the \ze~$=4.12$ quasar Q$0000-2619$ have been obtained in the
range $\lambda\lambda = 4880-8115$ \AA~with a resolution of 13 \kms~and
signal--to--noise ratio of $s/n=15-60$ per resolution element. The mode of the
distribution of the Doppler parameters for the \lya~lines is $\simeq 25$ \kms.
The fraction of lines with $10<b<20$ \kms~is 17\%. The Doppler values derived
from uncontaminated \lyb~lines are smaller than those obtained from the
corresponding Ly$\alpha$ lines, indicating the contribution of non saturated,
non resolved components in the \lya~profiles. The integrated UV background
estimated from the proximity effect is found to be $J \sim 7 \times 10^{-22}$
erg s$^{-1}$ cm$^{-2}$~Hz$^{-1}$ sr$^{-1}$. This value is consistent with
previous estimates obtained at a lower $z$, implying no appreciable redshift
evolution of the UVB up to $z=4$. 13 metal systems are identified, five of
which previously unknown. The analysis of the associated metal systems suggests
abundances generally below the solar value with an average [C/H] $\sim -0.5$.
This value is about one order of magnitude higher than that found in
intervening systems at about the same redshift. The analysis of the intervening
metal line systems has revealed in particular the presence of three optically
thin systems with $\log N_{HI}\sim 15$ showing associated CIV and SiIV
absorptions. In order to make the observed column densities consistent with
[Si/C] ratios lower than 10 times the solar value, it is necessary to assume a
large jump in the spectrum of the ionizing UV background beyond the HeII edge
($J_{912}/J_{228}\gsim 1000$). This result, if confirmed in other spectra at
the same redshift is suggestive of a possible dominance of a stellar ionizing
emissivity over the declining quasar one at $z > 3$.
