The four rings weren't meant to replace his Neural Frames, they were there to take what each Frame already did best and drag that specialty several steps farther than Grey's flesh could manage on its own. Once the chains were attached, those rings would also become the interfaces between Grey and the artificial Frames themselves, deciding how signals entered, exited, and were translated between the two.
But the fifth ring would be different. It had to be able to take inputs directly from Prometheus and coordinate everything happening in all the other rings at the same time.
Grey had spent a lot of time thinking about how he started his loops initially. Logically speaking, before he got Prometheus, his deaths should have just been deaths, and that would be the end of it, but there were several times Prometheus had broken the rules just for him.
