It doesn't work like that. The Ethernet connection needs to come from the router to the PS4. So, if the router is upstairs and the PS4 is downstairs, you need to figure out how to run the Ethernet cable between each location.
You would have the same issue in reverse, so if your router was situated downstairs near the PS4, you would then have the problem of running the Ethernet cable upstairs to the PC.
The only other option I can think of is to use a couple of Powerline adapters to send a wired connect along your power cables. To do this, plug one in near the router and connect an Ethernet cable to it from the router, then plug the other adapter in near the PS4 and connect an Ethernet to it from the PS4. Get passthrough ones, then you don't lose the plug socket consumed by the adapters.
SH2 modem mode | AC86U router | AC68U node