Hm, the main tricky bit with adding a 5th player is how the 4 card suits get used for dividing up narration rights between players. A 5th player wouldn’t be able to have sole authority over a suit.
I've been thinking about a way around that, and one idea that might work would be to have the 5th player take the role of a “Spirit,” along the lines of a eudaemon from Greek mythology. Here's how I could see that working:
The Spirit can add incidental detail to any scene, and whisper advice & opinions to the Protagonist/primary player. When a new set of 3 tiles are laid out, the Spirit can pick one to save (or to swap for their saved tile) *before* the primary player makes any choices or saves any tile. The Spirit can offer their saved tile to the player just like the Fates can. If the primary player does choose the Spirit’s tile, the Spirit gets to narrate it as if they had authority over that tile’s suit.
Whoever plays the Spirit might also get first dibs on being the primary player next, if they end up having less to do while playing as the Spirit.
If you end up trying that out, let me know how it goes!