This goes under unsubstantiated rumor so don't get too excited (or annoyed). An accidental hashtag on Paramount Pictures Instagram account combined with comments from MichaelBay.com admin Nelson suggests that maybe Megan Fox returns in some capacity for Transformers 4. The since pulled picture had a hashtag of #MeganFox with an image of the Lamborghini Aventador LP 700-4 Coupe. When asked about this error Nelson at first thought it was fake because the post is more then 140 characters but when told it was on Instagram he said "My bad. Someone's in trouble. It's gonna be a long week."
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 MichaelBay.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.