The fourth member will have to be the ring leader so it will have to be someone already established. I wouldn't mind seeing Christian be behind them with the reason being that all the people they have gone after have wronged Christian. Ryback and Cena both have been given pushes that WWE never even dreamed of giving Christian. Sheamus injured Christian. Randy Orton defeated Christian for his World Heavyweight Championship just two days after winning it in a ladder match. The Miz cheated to beat Christian for the Intercontinental Championship. Kane has had multiple rivalries with Christian and Daniel Bryan went after the Shield when they went after Kane.
I like them as three for now, but if they were to add a fourth member, I'd want it to be Kassius Ohno. He should have been brought up A LOT sooner, and having him as a member of Shield would be good for him. Either that, or I'd want him to fued with Cesaro for the US title, but that's just me.