  We use 29 z>4 quasar spectra to build a homogeneous sample of high-redshift
CIV absorbers. We use these data to calculate the number density, n(z), of
W_rest(CIV)>0.30 A CIV doublets. We find that n(z) increases with time from z ~
4.5. In addition, W_rest(CIV)>0.15 A CIV systems are more numerous than the
former at all redshifts. On the contrary, n(z) of W_rest(MgII)>0.30 A MgII
doublets decreases with time, in agreement with results from studies of LLS
number densities. Below z<3, none of all the classes of absorbers show signs of
evolution. We interpret this as the formation of galactic envelopes from
smaller halos. Furthermore, the doublet ratio DR=W(CIV 1548)/W(CIV 1550) is
found to decrease with time, a signature of the increase of the mean CIV column
density. Finally, the W(SiIV 1393)/W(CIV 1548) ratios in strong absorbers
(W_rest(CIV 1548) > 0.50 A) is found to be approximately constant from z=3.5 to
z=2.5 and then to decrease with time. This result suggests that the highest
column density absorbers are not sensitive to changes in the ionization
continuum.
