We use 29 z > 4 quasar spectra to build a homogeneous sample of high-redshift C IV absorbers .
We use these data to calculate the number density , n ( z ) , of W _ { rest } ( { C \textsc { IV } } ) > 0.30 Å C IV doublets .
We find that n ( z ) increases with time from z \sim 4.5 .
In addition , W _ { rest } ( { C \textsc { IV } } ) > 0.15 Å C IV systems are more numerous than the former at all redshifts .
On the contrary , n ( z ) of W _ { rest } ( { Mg \textsc { II } } ) > 0.30 Å Mg II 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 ( { C \textsc { IV } } 1548 ) / W ( { C \textsc { IV } } 1550 ) is found to decrease with time , a signature of the increase of the mean C IV column density .
Finally , the W ( Si IV 1393 ) / W ( C IV 1548 ) ratios in strong absorbers ( W _ { rest } ( { C \textsc { IV } } 1548 ) \geq 0.50 Å ) 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 .