The space density of z > 4 blazars

High-redshift blazars are an important class of active galactic nuclei (AGN) that can provide an independent estimate of the supermassive black hole mass function in high-redshift radio-loud AGN without the bias due to absorption along the line of sight. Using the Cosmic Lens All Sky Survey (CLASS), we built a complete radio flux-limited sample of high-redshi ft (z > 4) blazars suitable for statistical studies. By combining dedicated optical observations and the SDSS spectroscopic database, we obtained a sample of 26 blazar candidates with a spectroscopic redshift above 4. On the basis of their radio spectrum, we distinguish between blazars and QSO with a Gigahertz Peaked Spectrum (GPS) like spectrum. Out of the 18 confirmed blazars 14 constitute a completely identified, flux-limited sample down to a magnitude of 21 (AB). Using this complete sample, we derive a space density of blazars with 4 < z < 5.5 of rho = 0.13(-0.03)(+0.05) Gpc(-3). This is the first actual estimate of the blazar space density in this range of redshift. This value is in good agreement with the extrapolation of the luminosity function and cosmological evolution based on a sample of flat-spectrum radio quasars selected at lower redshifts and it is consistent with a cosmological evolution peaking at z similar to 2 similar to radio-quiet QSO. We do not confirm, instead, the presence of a peak at z similar to 4 in the space density evolution, recently suggested using an X-ray selected sample of blazars. It is possible that this extreme peak of the evolution is present only among the most luminous blazars.