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We already know that Megan Fox and Michael Bay repaired their relationship enough that she was able to get a starring role in the live action Teenage Mutant Ninja Turtles that Bay is executive producer on and is currently filming for a June 6, 2014 release date. The TMNT filming would have provided the perfect cover for a secret cameo role. If Fox has a larger role, I think the TMNT filming is nearing the end of principle photography so she would be available for TF4 as it films over the summer. It is also possible that the person that posted the picture is not really a Transformers fan and wasn't aware she was no longer part of the franchise. As always time will tell. (via TFW2005)
Update: The Shoot for the Edit twitter feed run by MB.com admin Nelson provided a simply "No" when asked if she was returning. Later he tweeted an article from April where Michael Bay said Fox was not returning to the franchise. So while this could be a trick...I just don't seem them bothering. To be blunt Fox doesn't have the star power to be worth the trouble nor would the "surprise" of an appearance accomplish anything useful fan service of value like the Wolverine cameo did in X-Men First Class.