What is the difference between a NEMA 4 and a NEMA 4X rated enclosure ?

Both NEMA 4 and 4X enclosures are rated primarily for outdoor use , providing a degree of protection against windblown dust and rain,  splashing and hose directed water while undamaged by the formation of ice on the enclosure . The NEMA 4X provides an additional level of  protection against corrosion.