Mission impossible and I mean really impossible at this point.
You cannot mount a network share on the PS4 and you cannot make it work as a drive where the games are going to be loaded from. The only remotely close thing is the remote PKG installer that loads PKGs from a device via network instead of a connected USB HDD (games are still installed to the PS4 HDD). It however isn't what you want.
The other thing is... Your Raspberry Pi is slow as hell. Old USB 2.0 interfaces on USB 2.0 lanes that are shared between all USB ports, the Ethernet NIC and some other interfaces. Giving you just max. 480 Mbps which in reality is around 20 to 30 MB/s... Which is more than a few times slower than using USB 3.0 HDD via USB on the PS4 directly. It's a I/O disaster.