You would need a receiver capable of playing 2 audio streams at the same time. This is VERY easy for analog audio(diy easy), but digital requires more work.
You may be able to workaround this by just connecting a line out or headphone out jack from your tv to your computer's line in and using your operating system to "listen" to the line in. This would play the PS4 game audio on your computers speakers at the same time as your sound.
Clearly this line-in method removes surround sound, but some upmixing may be possible on the computer(this is software dependent).
You could use a DAC to turn the PS4's audio(optical or maybe usb) to analog audio and mix in with the computers own analog audio to be sent to the same speakers. This is more of a DIY idea. It can work well, but can also require some tweaking and analog audio[even digital systems can get this, but optical is immune because it has no shared ground just light] can get ground loops leading to a humming sound[warning of this because in a home entertainment system you may have many devices connecting all sharing common grounds that may not always be at the same potential].
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