The rank of SP4 once denoted the fourth grade of the specialist rank in the U.S. Army. The term was abandoned in favor of "SPC," the only specialist rank, and is roughly equivalent to the rank of corporal.
The nine specialist ranks were introduced to the U.S. Army in 1955 and 1958 to provide a full career path for enlisted soldiers with extensive technical knowledge. Soldiers in the Armed Forces with such knowledge were likely to be utilized in a technical capacity throughout their military careers, leaving them too specialized to advance through the normal ranks of troop command. The SP ranks provided prestige and pay incentives for career service. Over the years, SP ranks were abolished and reclassified under the rank “SPC.”