  We report ALMA detections of [CII] and dust continuum in Az9, a
multiply-imaged galaxy behind the Frontier Field cluster MACSJ0717.5+3745. The
bright [CII] emission line provides a spectroscopic redshift of z = 4.274. This
strongly lensed (mu = 7 +/- 1) galaxy has an intrinsic stellar mass of only 2e9
Msun and a total star formation rate of 26 Msun/yr (~80% of which is dust
obscured). Using public magnification maps, we reconstruct the [CII] emission
in the source plane to reveal a stable, rotation-dominated disk with V/sigma =
5.3, which is > 2x higher than predicted from simulations for similarly
high-redshift, low-mass galaxies. In the source plane, the [CII] disk has a
half-light radius of 1.8 kpc and, along with the dust, is spatially offset from
the peak of the stellar light by 1.4 kpc. Az9 is not deficient in [CII];
L[CII]/LIR = 0.0027 consistent with local and high redshift normal star forming
galaxies. While dust-obscured star formation is expected to dominate in higher
mass galaxies, such a large reservoir of dust and gas in a lower mass disk
galaxy 1.4 Gyr after the Big Bang challenges our picture of early galaxy
evolution. Furthermore, the prevalence of such low-mass dusty galaxies has
important implications for the selection of the highest redshift dropout
galaxies with JWST. As one of the lowest stellar mass galaxies at z > 4 to be
detected in dust continuum and [CII], Az9 is an excellent laboratory in which
to study early dust enrichment in the interstellar medium.
