  We report the discovery of two radio-loud quasars with redshifts greater than
four; GB1428+4217 with z=4.72 and GB1713+2148 with z=4.01. This doubles the
number of published radio-selected quasars with z > 4, bringing the total to 4.
GB1428+4217 is the third most distant quasar known and the highest redshift
radio and X-ray source currently known. It has a radio flux density at 5GHz of
259 +/- 31 mJy and an optical magnitude of R approximately 20.9. The rest frame
absolute UV magnitude, M(nu=1450A), is -26.7 similar to that of the archetypal
radio-selected quasar 3C273 (z=0.158; M(nu=1450A)=-26.4). GB1428+4217 has a
tentative detection in ROSAT PSPC observations, which has been confirmed by
more recent ROSAT observations, described in a companion paper by Fabian et al.
Both quasars were discovered during the CCD imaging phase of an investigation
into the evolution of the space density of radio-loud quasars at high redshift.
Combined with our earlier survey results these objects give a lower limit on
the space density of quasars with radio power P(5GHz) > 5.8 x 10^{26} W/Hz/sr
between z=4 and z=5 of 1.4 +/- 0.9 x 10^{-10} Mpc^{-3}. This can be compared to
2.9 +/- 0.2 x 10^{-10} Mpc^-3 at z=2 from Dunlop & Peacock (1990) for
flat-spectrum sources of the same luminosity.
