  Despite the importance of the dusty star-forming galaxies (DSFGs) at $z$>2
for understanding the galaxy evolution in the early Universe, their stellar
distributions traced by the near-IR emission were spatially unresolved until
the arrival of the JWST. In this work we present, for the first time, a
spatially-resolved morphological analysis of the rest-frame near-IR
(~1.1-3.5$\mu$m) emission in DSFGs traced with the JWST/MIRI. In particular, we
study the mature stellar component for the three DSFGs and a Lyman-break galaxy
(LBG) present in an overdensity at $z$=4.05. Moreover, we use MIRI images along
with UV to (sub)-mm ancillary photometric data to model their SEDs and extract
their main physical properties. The sub-arcsec resolution MIRI images have
revealed that the stellar component present a wide range of morphologies, from
disc-like to compact and clump-dominated structures. These near-IR structures
contrast with their UV emission, which is usually diffuse and off-centered. The
SED fitting analysis shows that GN20 dominates the total SFR with a value ~2500
$M_\odot$yr$^{-1}$ while GN20.2b has the highest stellar mass in the sample
($M_*$~2$\times$10$^{11}$ $M_\odot$). The two DSFGs classified as LTGs (GN20
and GN20.2a) show high specific SFR (sSFR>30 Gyr$^{-1}$) placing them above the
star-forming main sequence (SFMS) at z~4 by >0.5 dex while the ETG
(i.e.,GN20.2b) is compatible with the high-mass end of the main sequence. When
comparing with other DSFGs in overdensities at $z$~2-7 we observe that our
objects present similar SFRs, depletion times and projected separations.
Nevertheless, the effective radii computed for GN20 and GN20.2a are up to two
times larger than those of isolated galaxies observed in CEERS and ALMA-HUDF at
similar redshifts. We interpret this difference in size as an effect of rapid
growth induced by the dense environment.
