Hey Hukstable, your plant looks like it's the longer spined T. scopulicola form. Four ribs isn't unusual for the plant. My own had four ribs, but sadly switched to five last season.
I asked Prier if this plant was a scop x pach but he said that it was a pachanoi (woolunda monster pedro) that a friend had imported into australia in the early 90s. It looks like a long spine scop. Would be interesting to know where it came from. It is supposedly semi monstrose and morphs often. I hope it stays as 4 ribbed. Do you have any pics of yours?