  The D4000 spectral break index is one of the most important features in the
visible spectrum, as it is a proxy for stellar ages and is also used in galaxy
classification. However, its direct measurement has always been reserved to
spectroscopy. Here, we present a general method to directly measure the D4000
with narrow-band (NB) photometry; it has been validated using realistic
simulations, and then evaluated with PAUS NBs, cross-matched with VIPERS
spectra ($i_{\rm AB} < 22.5$, $0.562 < z < 0.967$). We also reconstruct the
D4000 with the SED-fitting code CIGALE; the use of PAUS NBs instead of broad
bands significantly improves the SED fitting results. For D4000$_{\rm n}$, the
direct measurement has $\rm \langle SNR \rangle \sim 4$, but we find that for
$i_{\rm AB}<21$ all direct D4000 measurements have $\rm SNR>3$. The CIGALE
D4000$_{\rm n}$ has $\rm \langle SNR \rangle \sim 20$, but underestimates the
error by $>$50\%. Furthermore, the direct method recreates well the D4000-SFR
relation, as well as the D4000-mass relation for blue galaxies (for red
galaxies, selection effects impact the results). On the other hand, CIGALE
accurately classifies galaxies into red and blue populations. We conclude that
the direct measurement of D4000 with narrow-band photometry is a promising tool
to determine average properties of galaxy samples, with results compatible with
spectroscopy.
