The c'tan are essentially the opposite of the chaos gods, since they operate within the rules of the physical universe in opposition to the warp. In a manner of speaking, they're the physical universes version of the four, though I'd say the C'tan united are a much bigger threat, especially if they had the Necrons behind them like they originally did. I'd say that pre nerf Necrons were without a doubt the strongest faction in Warhammer 40k, and were basically guaranteed to "win". But then that was retcon'd.