From the NEMA brief description document:
NEMA 4 - 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
(falling dirt and 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); and that will be undamaged by the external
formation of ice on the enclosure.