  We report on observations of Lyman-break galaxies (LBGs) selected from the
Great Observatories Origins Deep Survey at mean redshift z~4, 5 and 6, obtained
with FORS2 spectrograph at the ESO-VLT.This program has yielded spectroscopic
identifications for 114 galaxies of which 51 are at z~4, 31 at z~5, and 32 at
z~6. The adopted selection criteria are effective, identifying galaxies at the
expected redshift with minimal foreground contamination. Once selection effects
are properly accounted for, the rest-frame UV spectra of the higher-redshift
LBGs appear to be similar to their counterparts at z~3. As at z~3, LBGs at z~4
and z~5 show Lya both in emission and in absorption; when in absorption, strong
interstellar lines and red UV spectra are observed, a fact also measured at z~2
and 3. At z~6, sensitivity issues bias our sample towards galaxies with Lya in
emission; nevertheless, these spectra appear to be similar to their
lower-redshift counterparts. As in other studies at similar redshifts, we find
clear evidence that brighter LBGs tend to have weaker Lya emission lines. At
fixed rest-frame UV luminosity, the equivalent width of the Lya emission line
is larger at higher redshifts. At all redshifts where the measurements can be
reliably made, the redshift of the Lya emission line turns out to be larger
than that of the interstellar absorption lines,with a median velocity
difference DeltaV~400 km/s at z~4 and 5, consistent with results at lower
redshifts. This shows that powerful, large-scale winds are common at high
redshift.In general,there is no strong correlation between the morphology of
the UV light and the spectroscopic properties. However, galaxies with deep
interstellar absorption lines and strong Lya absorption appear to be more
diffuse than galaxies with Lya in emission.(abridged)
