Specialist 4, SP4, E4 â “ The Specialist is a somewhat unique rank. This is the only enlisted rank that does not bear chevrons or rockers. Instead, the Specialist rank looks like a solid shield with an eagle centered on it. The Specialist has been trained in a military occupational specialty (MOS), his Army job, and has some amount of experience in the service.
Corporal, CPL, E4 â “ The Corporal wears the insignia of two chevrons. The Corporal is the first rank in the US Army identified as a non-commissioned officer, or NCO. The Corporal and the Specialist both make the same amount of pay, but the Corporal outranks the Specialist because of his NCO status. Also, the Corporal is the first rank designated as a 'Junior non-commissioned officer', or Junior NCO.
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More info on Specialist.
http://en.wikipedia.org/wiki/Specialist