If you had a 3.5mm type of headset this could be done with an RCA audio combiner.
Since you have USB this becomes much more tricky.
If the headset software/driver supports it you can connect your PS4 to PC and hopefully it will output both sounds but again that depends completely on your headset and motherbaord audio software.
If PS4 is connected to TV then just use RCA to 3.5mm cable to go from audio out on TV (or monitor wiht sound) and then conneect to line-in on your PC.
If you jsut have HDMI going to monitor with no audio then you will need something like this to extract audio from HDMI first...