Supoena on page 3 of this file
The person posting was Roberto Grijalva, a prison guard working at the prison where Epstein was jailed. His name is listed on page 88 of the above document.
See page 851 of this other pdf with proof of his employment. This apparently did not result in him being fired, as an inmate sued him in 2023 for use of excessive force.
I think the chain was something like get IP from 4chan -> tie IP to device with Apple -> get phone number for device from AT&T -> they track the shit out of your phone so it’s over. So it is kinda anonymous for people that can’t send out three subpoenas in an afternoon