  Using empty-field `Quick Look' images from the first two epochs of the VLA
Sky Survey (VLASS) observations, centred on the positions of $\sim3700$
individually radio-non-detected active galactic nuclei (AGNs) at $z\ge4$, we
performed image stacking analysis to examine the sub-mJy emission at $3$ GHz.
We found characteristic monochromatic radio powers of $P_\mathrm{char}=(2-13)
\times 10^{24}$ W Hz$^{-1}$, $P_\mathrm{char}=2\times10^{24}-1.3\times10^{25}$
W Hz$^{-1}$, indicating that AGN-related radio emission is widespread in the
sample. The signal-to-noise ratios of the redshift-binned median stacked maps
are between $4-6$, and we expect that with the inclusion of the yet to be
completed third-epoch VLASS observations, the detection limit defined as
signal-to-noise ratio $\mathrm{SNR}\ge6$ could be reached, and the redshift
dependence can be determined. To obtain information on the general spectral
properties of the faint radio emission in high-redshift AGNs, we confined the
sample to $\sim3000$ objects covered by both the VLASS and the Faint Images of
the Radio Sky at Twenty-centimeters (FIRST) survey. We found that the flux
densities from the median stacked maps show a characteristic spectral index of
$\alpha^*=-0.30\pm0.15$, which is in agreement with the median spectral index
of the radio-detected $z\ge4$ AGNs from our high-redshift AGN catalogue. The
three-band mid-infrared colour--colour diagram based on Wide-field Infrared
Survey Explorer observations provides further support regarding the AGN
contribution to the radio emission in the sub-mJy sample.
