  We examine the morphological and kinematical properties of SPT-2147, a
strongly lensed, massive, dusty, star-forming galaxy at $z = 3.762$. Combining
data from JWST, HST, and ALMA, we study the galaxy's stellar emission, dust
continuum and gas properties. The imaging reveals a central bar structure in
the stars and gas embedded within an extended disc with a spiral arm-like
feature. The kinematics confirm the presence of the bar and of the regularly
rotating disc. Dynamical modeling yields a dynamical mass, ${M}_{\rm dyn} =
(9.7 \pm 2.0) \times 10^{10}$ ${\rm M}_{\odot}$, and a maximum rotational
velocity to velocity dispersion ratio, $V / \sigma = 9.8 \pm 1.2$. From
multi-band imaging we infer, via SED fitting, a stellar mass, ${M}_{\star} =
(6.3 \pm 0.9) \times 10^{10}$ $\rm{M}_{\odot}$, and a star formation rate,
${\rm SFR} = 781 \pm 99$ ${\rm M_{\odot} yr^{-1}}$, after correcting for
magnification. Combining these measurements with the molecular gas mass, we
derive a baryonic-to-total mass ratio of ${M}_{\rm bar} / {M}_{\rm dyn} = 0.9
\pm 0.2$ within 4.0 kpc. This finding suggests that the formation of bars in
galaxies begins earlier in the history of the Universe than previously thought
and can also occur in galaxies with elevated gas fractions.
