  The Mg II h&k lines are the prime chromospheric diagnostics of NASA's
Interface Region Imaging Spectrograph (IRIS). In the previous papers of this
series we used a realistic three-dimensional radiative magnetohydrodynamics
model to calculate the h&k lines in detail and investigated how their spectral
features relate to the underlying atmosphere. In this work, we employ the same
approach to investigate how the h&k diagnostics fare when taking into account
the finite resolution of IRIS and different noise levels. In addition, we
investigate the diagnostic potential of several other photospheric lines and
near-continuum regions present in the near-ultraviolet (NUV) window of IRIS and
study the formation of the NUV slit-jaw images. We find that the instrumental
resolution of IRIS has a small effect on the quality of the h&k diagnostics;
the relations between the spectral features and atmospheric properties are
mostly unchanged. The peak separation is the most affected diagnostic, but
mainly due to limitations of the simulation. The effects of noise start to be
noticeable at a signal-to-noise ratio (S/N) of 20, but we show that with noise
filtering one can obtain reliable diagnostics at least down to a S/N of 5. The
many photospheric lines present in the NUV window provide velocity information
for at least eight distinct photospheric heights. Using line-free regions in
the h&k far wings we derive good estimates of photospheric temperature for at
least three heights. Both of these diagnostics, in particular the latter, can
be obtained even at S/Ns as low as 5.
