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We applied image stacking on empty-field Faint Images of the Radio Sky atTwenty-Centimeters (FIRST) survey maps centred on optically identifiedhigh-redshift quasars at $z\geq4$ to uncover the hidden $\mu$Jy radio emissionin these active galactic nuclei (AGN). The median stacking procedure for thefull sample of $2229$ optically identified AGN uncovered an unresolved pointsource with an integrated flux density of 52$~\mu$Jy, with a signal-to-noiseratio $\sim10$. We co-added the individual image centre pixels to estimate thecharacteristic monochromatic radio power at $1.4~$GHz considering variousvalues for the radio spectral index, revealing a radio population with$P_\mathrm{1.4GHz}\sim10^{24}~$W Hz$^{-1}$. Assuming that the entire radioemission originates from star-forming (SF) activity in the nuclear region ofthe host galaxy, we obtained an upper limit on the characteristic starformation rate, $\sim4200~$M$_\odot~$yr$^{-1}$. The angular resolution of FIRSTimages is insufficient to distinguish between the SF and AGN origin of radioemission at these redshifts. However, a comparison with properties ofindividual sources from the literature indicates that a mixed nature is likely.Future very long baseline interferometry radio observations and ultra-deepSquare Kilometre Array surveys are expected to be sensitive enough to detectand resolve the central $1-10~$kpc region in the host galaxies, and thusdiscriminate between SF and AGN related emission.

Unveiling the weak radio quasar population at z $\geq$ 4