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 \pm 31 mJy and an optical magnitude of R \sim 20.9 .
The rest frame absolute UV magnitude , M _ { \nu } ( 1450 \AA ) , is - 26.7 similar to that of the archetypal radio-selected quasar 3C273 ( z = 0.158 ; M _ { \nu } ( 1450 \AA ) = -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 _ { 5 GHz } > 5.8 \times 10 ^ { 26 } WHz ^ { -1 } sr ^ { -1 } between z = 4 and z = 5 of 1.4 \pm 0.9 \times 10 ^ { -10 } Mpc ^ { -3 } .
This can be compared to 2.9 \pm 0.2 \times 10 ^ { -10 } Mpc ^ { -3 } at z = 2 from Dunlop & Peacock ( 1990 ) for flat-spectrum sources of the same luminosity .