The idea of adding another member to The Shield is idiotic if you ask me. They've been going perfectly, and no matter how they do it or the level of the performer they choose (which has to be someone from NXT), it will upset the balance these three have. There is legitimately zero reason to add anyone to the team, from a kayfabe perspective (The Shield are undefeated and that's on those three merits alone, adding another member would be pointless. Plus, having a secret member or someone as the leader, destroys like the biggest fundamentals of the team itself, them being in it for themselves more than anything else. They wouldn't have a secret member since having a secret member doesn't benefit them at all, and that member himself hiding out won't benefit them, especially since it's The Shield that's always on the offensive. Having a leader defeats the purpose entirely for reasons I'm not going to get into due to being obvious. And no, Heyman doesn't count, at least not now. He hired them to do a job but that job undoubtedly benefited the team and their is no evidence to support that Heyman hired them to do all the other attacks they've done after or during) or even from a booking perspective (the only purpose I can think of is if they want the new member to be the reason for the break-up but that is both stupid [as if he was causing the rift, I'd like to think that based off previous booking, The Shield is smart enough to realize that and just kick his ass out of the team] and a perfect example of bad writing. For both the reason that it's stupid but also because it's an unrelated element from the past few months of booking [and let's be honest, The Shield probably won't be lasting past the Summer which makes it worse] and except arguably that Raw moment in the locker room, there has been no alluding or foreshadowing to a fourth member of The Shield. In short, it's pretty close to a Deus ex Machina and again, The Shield have zero reason to add a fourth member, so it will be pretty obvious that the person was added only to end the team. If you're adding someone just because it worked for the original [and should be only] three then that's laziness personified and will definitely come across as force).
That all said, I'm not actually worried about them adding a fourth member. From memory, not a single thing a WWE article has teased has ever actually happened. I remember they teased resurrecting the Hardcore title for example. So fortunately, I don't see this happening.