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 Å), is −26.7, similar to that of the archetypal radio‐selected quasar 3C273 [ z =0.158; M v (1450 Å)=−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 >5.8×10 26 W Hz −1 sr −1 between z =4 and z =5 of 1.4±0.9×10 −10 Mpc −3 . This can be compared to 2.9±0.2×10 −10 Mpc −3 at z =2 from Dunlop & Peacock for flat‐spectrum sources of the same luminosity.