  The study of the space density of bright AGNs at $z>4$ has been subject to
extensive effort given its importance for the estimate of the cosmological
ionizing emissivity and growth of supermassive black holes. In this context we
have recently derived high space densities of AGNs at $z\sim 4$ and
$-25<M_{1450}<-23$ in the COSMOS field from a spectroscopically complete
sample. In the present paper we attempt to extend the knowledge of the AGN
space density at fainter magnitudes ($-22.5<M_{1450}<-18.5$) in the $4<z<6.1$
redshift interval by means of a multiwavelength sample of galaxies in the
CANDELS GOODS-South, GOODS-North and EGS fields. We use an updated criterion to
extract faint AGNs from a population of NIR (rest-frame UV) selected galaxies
at photometric $z>4$ showing X-ray detection in deep Chandra images available
for the three CANDELS fields. We have collected a photometric sample of 32 AGN
candidates in the selected redshift interval, six of which having spectroscopic
redshifts. Including our COSMOS sample as well as other bright QSO samples
allows a first guess on the shape of the UV luminosity function at $z\sim 4.5$.
The resulting emissivity and photoionization rate appear consistent with that
derived from the photoionization level of the intergalactic medium at $z\sim
4.5$. An extrapolation to $z\sim 5.6$ suggests an important AGN contribution to
the IGM ionization if there are no significant changes in the shape of the UV
luminosity function.
