It is to enable you to have your source (Bluray or SKy Q) connected to the SB and then connect SB to TV - SB plays Audio and 4K picture goes to TV. Some that support 4K may not support HDR or Dolby Vision, so worth checking if you want that too.
If you connect the PS4 to TV, then you can use Optical output to most SB. The only limitation then is that Optical will not pass some HD sound formats (such as Atmos). To use HDMI output from TV then that is using ARC. Suggest you read this: HDMI ARC and HDMI eARC: everything you need to know