Oh, you bought the UPS. Well, good to know it can handle the PS4. About the in-built battery, it won't last more than 15 to 20 mins, but that should be ample of time for you to save your game and shut it down.
Hi,
If you bought the one that I have (Luminous Zelios 1100+) then you just need one battery. If the batteries work just fine, then you can simply hook one of those up to it. No need to buy new.
Also, to answer your previous question, getting a simple UPS would've been fine too, like the one which @Bauji got. That would've given you 15 to 20 mins to save your game and shut down.
Lemme know if you have any more question
PS: I don't login to IVG much these days but I'll keep checking in every now and then