Can a E4 Promotable reenlist for an e5 slot?

SGT (Join to see) Unless things changed, you don't re-up for a slot, you re-up for a specific installation, or to change MOS, etc. You can always serve up or down based on your grade, but you' don't make that decision, and we don't for sure determine in what unit we land when we get to x,y, or z installation.

Its almost impossible to obtain pin-point orders at that level. However, your branch might be able to answer that question. Its not really that simple though.

I know you can reclass to certain MOS's such as 35L as a SPC(P) and upon completion of AIT, you will be promoted to SGT.