Discovery of radio-loud quasars with z=4.72 and z=4.010

We report the discovery of two radio-loud quasars with redshifts greater than 4: 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 5 GHz of 259+/-31 mJy and an optical magnitude of R~20.9. The rest frame absolute UV magnitude, M_v(1450 A), is -26.7, similar to that of the archetypal radio-selected quasar 3C273 [z=0.158 M_v(1450 A)=-26.4]. GB1428+4217 is tentatively detected 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_5 GHz>5.8x10^26 W Hz^-1 sr^-1 between z=4 and z=5 of 1.4+/-0.9x10^-10 Mpc^-3. This can be compared to 2.9+/-0.2x10^-10 Mpc^-3 at z=2 from Dunlop & Peacock for flat-spectrum sources of the same luminosity.