An analog 3.5mm splitter box won't work with a PS4 because they don't have an analog output; only HDMI and fiber optic.
So you're saying that you have two monitors ( side by side by the sound ) and you want both your computer and your PS4 to output audio over the same device so you don't need to buy two sets of speakers?
The simplest solution to this is a headset. The PS4 can output game and chat audio through the controller, so plug whatever headphones or ear buds you want into the controller. If you have a four-pole connector, even something as simple as earbuds with an inline mic, you can use that as a headset to chat while playing. When you want audio from the computer, switch the headphones over there. If you're asking how to get audio from both over the same speaker/headphones at the same time, it's a more difficult solution.