  We post-process galaxies in the IllustrisTNG simulations with SKIRT radiative
transfer calculations to make predictions for the rest-frame near-infrared
(NIR) and far-infrared (FIR) properties of galaxies at $z\geq 4$. The
rest-frame $K$- and $z$-band galaxy luminosity functions from TNG are overall
consistent with observations, despite a $\sim 0.4\,\mathrm{dex}$
underprediction at $z=4$ for $M_{\rm z}\lesssim -24$. Predictions for the JWST
MIRI observed galaxy luminosity functions and number counts are given. We show
that the next-generation survey conducted by JWST can detect 500 (30) galaxies
in F1000W in a survey area of $500\,{\rm arcmin}^{2}$ at $z=6$ ($z=8$). As
opposed to the consistency in the UV, optical and NIR, we find that TNG,
combined with our dust modelling choices, significantly underpredicts the
abundance of most dust-obscured and thus most luminous FIR galaxies. As a
result, the obscured cosmic star formation rate density (SFRD) and the SFRD
contributed by optical/NIR dark objects are underpredicted. The discrepancies
discovered here could provide new constraints on the sub-grid feedback models,
or the dust contents, of simulations. Meanwhile, although the TNG predicted
dust temperature and its relations with IR luminosity and redshift are
qualitatively consistent with observations, the peak dust temperature of $z\geq
6$ galaxies are overestimated by about $20\,{\rm K}$. This could be related to
the limited mass resolution of our simulations to fully resolve the porosity of
the interstellar medium (or specifically its dust content) at these redshifts.
