TL,DR: to find a Jailbreakable PS4, eBay is your best bet.
PS4s running firmware 9.00 or lower can be jailbroken.
PS4 9.00 Can be Jailbroken – Downgrade not possible – Higher Firmwares unhackable
At the time of writing, there is no clear signal from the hacking community that any Jailbreak is planned for recent PS4 firmwares. It’s not to say that nothing’s being worked on (hackers can be very secretive), but it could still be a while before any Jailbreak is released for firmwares 9.03 and above.
furthermore, there is no* way to downgrade a PS4 at this point, so if you’re looking to Jailbreak your PS4, your best bet is to get your hands on a low firmware console.
Because of the points above, I truly believe that your best bet if you want a Jailbroken PS4, is to buy a console running 9.00 or below.
Where to get your hands on a PS4 9.00
We used to have big lists of “bundles” that you could find on major retailers, to help you find lower firmware consoles. Unfortunately, these bundles are a thing of the past, and, although you can still find them sometimes, it’s for very inflated prices.
You won’t find lower-firmware PS4s on regular retailers anymore, but second-hand marketplaces like eBay have them for reasonable prices. (Between $200 and $300 at the time of writing).
* It is technically possible to downgrade a PS4 but requires specialized hardware, as well as a backup of the console’s syscon from a lower firmware. If you have that handy, you most likely don’t need to me to tell you where and how to find a PS4.