Since you are going thru the TV, the TV will only be passing 2 channel sound via it's optical (also the TV only is recognized as a 2 channel device)
The solution is to connect the optical cable directly from the PS4 to the receiver. On your PS4, go into the "Settings" then "Sound and Screen", then "Audio Output Settings"
Change the "Primary Output" to "Digital Out(Optical)" and change the "Audio Format(Priority)" to DTS or Dolby (I prefer DTS myself).
This will make the PS4 output sound to both the TV and your receiver at the same time, so if you still want to use only the TV speakers for sound you can.