  We use the GLASS-JWST Early Release Science NIRCam parallel observations to
provide a first view of the UV continuum properties of NIRCam/F444W selected
galaxies at 4<z<7. By combining multiwavelength NIRCam observations, we
constrain the UV continuum slope for a sample of 401 galaxies with stringent
quality controls. We find that >99% of the galaxies are blue star-forming
galaxies with very low levels of dust (Avbeta~0.01+/-0.33). We find no
statistically significant correlation for UV slope with redshift or UV
magnitude. However, we find that in general galaxies at higher redshifts and
fainter UV magnitudes have steeper UV slopes. We find a statistically
significant correlation for UV slope with stellar mass, with galaxies with
higher stellar mass showing shallower UV slopes. Individual fits to some of our
galaxies reach the bluest UV slopes of beta~-3.1 allowed by stellar population
models used in this analysis. Therefore, it is likely that stellar population
models with higher amount of Lyman continuum leakage, AGN effects, and/or
Population III contributions are required to accurately reproduce the rest-UV
and optical properties of some of our bluest galaxies. This dust-free early
view confirms that our current cosmological understanding of gradual mass +
dust buildup of galaxies with cosmic time is largely accurate to describe the
~0.7-1.5 Gyr age window of the Universe. The abundance of a large population of
UV faint dust-poor systems may point to a dominance of low-mass galaxies at z>6
playing a vital role in cosmic reionization.
