  In this fourth paper of a series on the precision of ages of stellar
populations obtained through the full-spectrum fitting technique, we present a
first systematic analysis that compare the age, metallicity and reddening of
star clusters obtained from resolved and unresolved data (namely
colour-magnitude diagrams (CMDs) and integrated-light spectroscopy) using the
same sets of isochrones. We investigate the results obtained with both Padova
isochrones and MIST isochrones. We find that there generally is a good
agreement between the ages derived from CMDs and integrated spectra. However,
for metallicity and reddening, the agreement between results from analyses of
CMD and integrated spectra is significantly worse. Our results also show that
the ages derived with Padova isochrones match those derived using MIST
isochrones, both with the full spectrum fitting technique and the CMD fitting
method. However, the metallicity derived using Padova isochrones does not match
that derived using MIST isochrones using the CMD method. We examine the ability
of the full-spectrum fitting technique in detecting age spreads in clusters
that feature the extended Main Sequence Turn Off (eMSTO) phenomenon using
two-population fits. We find that 3 out of 5 eMSTO clusters in our sample are
best fit with one single age, suggesting that eMSTOs do not necessarily
translate to detectable age spreads in integrated-light studies.
