  We analyze the optical-near infrared spectra of 33 quasars with redshifts
3.9<z<6.4 to investigate the properties of dust extinction at these cosmic
epochs. The SMC extinction curve has been shown to reproduce the dust reddening
of most quasars at z<2.2; we investigate whether this curve also provides a
good description of dust extinction at higher redshifts. We fit the observed
spectra with synthetic absorbed quasar templates obtained by varying the
intrinsic slope (alpha), the absolute extinction (A3000), and by using a grid
of empirical and theoretical extinction curves. We find that seven quasars in
our sample are affected by substantial extinction (A3000 >0.8), and
characterized by very steep intrinsic slopes (alpha<-2.3). All of the
individual quasars require extinction curve deviating from that of the SMC,
with a tendency to flatten at lambda<2000 A (in the rest frame of the source).
We obtain a mean extinction curve at z>4, both by performing a simultaneous fit
of all quasars and by averaging the extinction curves inferred for individual
quasars. In the case of broad absorption line quasars the mean extinction curve
deviates from the SMC at a confidence level >95%. The difference between
extinction curves in quasars at z>4 and in quasars at lower redshift is
indicative of either a different dust production mechanism at high redshift, or
a different mechanism for processing dust into the ISM. We suggest that the
same transitions may also apply to normal, star-forming galaxies at z>4. In
particular, the observed change in the average spectral slope of galaxies at
z>4 may be partially ascribed to a variation in the extinction curve, rather
than a lower dust content at high redshift. In this scenario, the extinction
curve inferred at z>4 would imply a cosmic star-formation corrected for dust
attenuation a factor of ~2 higher than estimated in the past.
