From the NEMA brief description document:
Type 4X - Enclosures constructed for either indoor or outdoor use to provide a degree
of protection to personnel against access to hazardous parts; to provide a degree of
protection of the equipment inside the enclosure against ingress of solid foreign objects
(windblown dust); to provide a degree of protection with respect to harmful effects on
the equipment due to the ingress of water (rain, sleet, snow, splashing water, and hose
directed water); that provides an additional level of protection against corrosion; and
that will be undamaged by the external formation of ice on the enclosure.