  We report the results of XMM-Newton observations of the X-ray bright,
radio-loud blazar GB B1428+4217 at a redshift of z=4.72. We confirm the
presence of soft X-ray spectral flattening at energies <0.7 keV as reported in
previous ROSAT and BeppoSAX observations. At hard X-ray energies the spectrum
is consistent with a power-law although we find the spectral slope varied
between both XMM-Newton observations and is also significantly different from
that reported previously. Whilst we cannot rule-out intrinsic cold absorption
to explain the spectral depression, we favour a dust-free warm absorber. Cold
absorption requires a column density ~1.4-1.6 x 10^22 cm^-2 whilst a warm
absorber could have up to ~10^23 cm^-2 and an ionization parameter ~10^2. The
spectrum of GB B1428+4217 shows remarkable parallels with that of the z=4.4
blazar PMN J0525-3343, in which the available evidence is also most consistent
with a warm absorber model.
