  We report on the precise determination of both the band gap E$_\text{g}$, and
the characteristic energy $U$ of the band tail of localized defect states, for
monocrystalline Ag$_2$ZnSnSe$_4$. Both photoluminescence excitation and
time-resolved photoluminescence studies lead to E$_\text{g} = 1223\pm3$ meV,
and $U = 20\pm3$ meV, at 6 K. The interest of the methodology developed here is
to account quantitatively for the time-resolved photoluminescence and
photoluminescence excitation spectra by only considering standard textbook
density of states, and state filling effects. Such an approach is different
from the one most often used to evaluate the energy extent of the localized
states, namely by measuring the energy shift between the photoluminescence
emission and the excitation one -- the so-called Stokes shift. The advantage of
the present method is that no arbitrary choice of the low power excitation has
to be done to select the photoluminescence emission spectrum and its peak
energy.
