HIGH-RESOLUTION RADIO IMAGING OF THE EXTREMELY DISTANT QUASAR-1251-407, QUASAR-1351-018, QUASAR-1354-174 AND QUASAR-1508+572

We present results of first epoch high resolution imaging observations of four extremely distant quasars. The sample includes the only two r adio-loud quasars at redshift z > 4 known to the date of observations, 1251-407 (z = 4.46) and 1508+572 (z = 4.30). The 5 GHz observations o f 1251-407, 1351-018 (z = 3.71) and 1354-174 (z = 3.15) were made with the VLBA in January 1995. The source 1508+572 was observed with the E VN in May 1995. The milliarcsecond scale structures of these quasars a re dominated by compact cores. The two z > 4 quasars are unresolved on the baselines observed. We show that the apparently less prominent mi lliarcsecond scale extended radio structures in high redshift quasars than in their low redshift counterparts may be explained by the differ ence in spectral indices of the cores and jets.