We exploit the 7 Ms Chandra observations in the Chandra Deep Field-South 
(CDF-S), the deepest X-ray survey to date, coupled with CANDELS/GOODS-S data, 
to measure the total X-ray emission arising from 2076 galaxies at 3.5<z<6.5. 
This aim is achieved by stacking the Chandra data at the positions of 
optically selected galaxies. The stacking results place constraints on the 
BHAD associated with the known high-redshift galaxy samples, as well as 
constraining the SFRD at high redshift. We find that the X-ray emission 
from our sample is likely dominated by processes related to star formation. 
Our results show that low-rate mass accretion onto SMBHs in individually 
X-ray-undetected galaxies is negligible, compared with the BHAD measured for 
samples of X-ray detected AGN, for cosmic SMBH mass assembly at high redshift. 
We also place, for the first time, constraints on the faint-end of the AGN 
X-ray luminosity function (logL_X~42) at z>4, with evidence for fairly flat 
slopes. This is in contrast with recent findings which implied a significant 
contribution of AGN to the cosmic Reionization.