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We define a new complete sample of 13 optically-luminous radio quasarsM_AB(1450 Angstrom) < -26.9 mag and log P_1.4 GHz(W/Hz) > 25.7 with redshift3.8 < z < 4.5, obtained by cross-correlating the FIRST radio survey and the APMcatalogue of POSS-I. We measure the space density to be 1.0 +/- 0.3 /Gpc^3, afactor 1.9 +/- 0.7 smaller than the space density of similar quasars at z=2.Using a new measurement of the radio-loud fraction of quasars we find that atz=4 the total space density of quasars with M_AB(1450 Angstrom) < -26.9 is 7.4+/- 2.6/Gpc^3. This is a factor 1.8 +/- 0.8 less than the space density at z=2,found by the 2dF quasar survey. This (z=2)/(z=4) ratio, consistent with that ofthe radio-loud quasars, is significantly different from the ratio of about 10found for samples including lower-luminosity quasars. This suggests that thedecline of the space density beyond z=2 is slower for optically-luminousquasars than for less-luminous ones.Comment: 13 pages, 6 postscript figures, to be published in Astrophys.  Journal, July 2003 issu

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