How is 12N AIT for prior service?

Speaking generally, Soldiers who attend AIT for MOS-T are not considered as an "IET Soldier" (those going through AIT as part of their Initial Entry Training).

You'll be assigned to an IET unit, must complete all course requirements and comply with regulations/standards unless specifically noted otherwise. However, in most situations the guidelines to leadership are to treat you as permanent party of the same grade (i.e., in your case, an E-4 assigned to Fort Leonard Wood and not a "boot") when it comes to privileges and billeting. For example, if available and feasible, PS Soldiers (E1-E4) will be billeted in non-IET barracks.

Again, guidance for leadership is to treat you as a non-IET Soldier that is new to the Army, but that isn't written in stone. Circumstances may dictate otherwise (e.g., there isn't availability in non-IET barracks, so you would be billeted there) as well as be situational (e.g., you're not meeting course requirements so your Commander restricts your privileges), but generally you would be treated as appropriate to a Soldier or your rank and time in service.
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* TRADOC Regulation 350-6 (Enlisted Initial Entry Training Policies and Administration) - https://adminpubs.tradoc.army.mil/regulations/TR350-6.pdf
* Fort Leonard Wood MSCoE and FLW Standards Book - https://home.army.mil/wood/application/files/1817/0016/7884/FLW_Standard_Book_Final_Draft_16NOV.pdf