Decline of the Space Density of Quasars betweenz= 2 andz= 4

We de. ne a new complete sample of 13 optically luminous radio quasars [M(AB)(1450 Angstrom) < -26.9 mag and log P(1.4GHz)(W Hz(-1)) > 25.7] with redshift 3.8 < z < 4.5, obtained by cross-correlating the Faint Images of the Radio Sky at Twenty cm ( FIRST) radio survey and the Automatic Plate Measuring Facility catalog of POSS I. We measure the space density to be 1.0 +/- 0.3 Gpc(-3), a factor of 1.9 +/- 0.7 smaller than the space density of similar quasars at z approximate to 2 ( FIRST Bright Quasar Survey). Using a new measurement of the radio-loud fraction of quasars, we find that at z = 4 the total space density of quasars with M(AB)(1450 Angstrom) < -26.9 mag is 7.4 +/- 2.6 Gpc(-3). This is a factor of 1.8 +/- 0.8 less than the space density at z approximate to 2, found by the Two-Degree Field quasar survey. This z = 2/z = 4 ratio, consistent with that of the radio-loud quasars, is significantly different from the ratio of similar to 10 found for samples including lower luminosity quasars. This suggests that the decline of the space density beyond z approximate to 2 is slower for optically luminous quasars than for less luminous ones.