Source: https://fail0verflow.com/blog/2015/console-hacking-2015-liner-notes.html
So fail0verflow if you remember are the group who paved the way for Geohot to jailbreak the PS3. Now the same team has done it again with the PS4 and have a low level system hack (low level means they got into the very core of the system).
Although they haven't written a step by step guide or included proper instructions on how to do it, the clue is that the system OS which is based on the free software FreeBSD which has many exploits documented online.
So anyway, they got in and showed Linux in an early form working on PS4.
Homebrew starts here!