As you've probably read in several speculation posts on some websites, a fourth member of The Shield has been rumored. So why not a new speculation post? Seth Rollins posted today about The Shield on Twitter which got a response from NXT's Sami Zayn, the former El Generico:
Rollins: "Tearing down that wall. Brick by brick by brick. We don't need no thought control. #BelieveInTheShield #raw"
Zayn: "@WWERollins I believe in the Shield."
So take that for what you will but as of last word, there were no plans at the current time to add a fourth member to The Shield faction.