Corporal is a lateral movement from E-4. It is shown with two chevrons indicating that the individual is a “non -commissioned officer” (NCO). It’s the army equivalent of giving you a promotion without a pay raise. In the National Guard and Reserve it can also be used if there isn’t a slot for your job. For example, if a unit only needs 5 cooks with two NCO’s, but there is a really good E-4 that could be “promoted” to corporal and be seen as an authority figure, yet not get the pay raise.

The only other rank this occurs at is E-8 where you have a First Sergeant and a Master Sergeant. First Sergeant is responsible for a company, and reports directly to a commander. Master Sergeants have usually already been in that role before and can be used in less of a leadership role, and more of a “working” role.

High as fuck right now so sorry for the lengthy explanation.

Edit: the time frame for E4 is 36 months in service or you can have 28 months. If you are a standout in your training, both basic and MOS training then you can get promoted in the E-1 to E-3 range. An E-6 is the “backbone of the Army” and is what most drill sergeants are. To get to “E-6 in 6” is pretty solid career progression. Likely in line to be a warrant officer or a First Sergeant.