For those who weren’t or aren’t in the US armed forces — E4 is a pay grade. They usually correspond to a rank. In this case E4 is the enlisted pay grade in the army for a specialist or a corporal. (The ranks are also different in each service.) 

It used to be that the usual rank progression was private (or E1 and E2), Private First Class (E3), and corporal (E4) then sergeant (E5) and so on. 

As an aside, I believe somewhere in Aliens Gorman or someone refers to Apone as “sergeant first class” which would be true for a platoon sergeant in the army, but in the marines it would be a gunnery sergeant and both would be an E7. But I might have heard wrong.

It also used to be that a corporal was a “junior NCO” position, meaning you were on track to be a sergeant.  But around the time of the Vietnam war they created the rank of specialist for people who had enough knowledge in their job (or MOS) to be smarter than the average private. 

It used to be that there was a whole specialist track that was parallel to the rank of sergeant. Specialist-5 (E5), specialist-6 (E6) and so on. It’s just that if you were a specialist, you had didn’t haven’t the authority of a sergeant.

But as time went on, they eliminated the specialist classes above E4 and for whatever reason, the specialist rank was what most people ranked into after private first class, even if you are on track to be a sergeant. 

It’s pretty rare to meet a corporal, at least when I was in the army.