I believe the fourth member of the Shield is Zack Ryder. This can be his heel turn that the IWC has been talking about for months. Ryder has posted an instagram photo of him dumping his ring attire and also made a heel video of his web series. This would be a shocking event in WWE!
Zack Ryder is at an absolute low point right now and I think adding him to a rising group like the Shield would just dilute them. It might help Ryder, but it would kill the Shield