The tweet was a reply to a previous tweet from S4TE about bringing Megan Fox back and the reply was "No. Female lead has been cast." If true her role would be the daughter of Mark Wahlberg's unnamed character with Jack Reynor as her unnamed race car driving boyfriend. Her name came up last year several times for the role. Now this information really isn't official since the twitter feed isn't from Michael Bay himself but from his site admin who uses it to promote Bay's various projects. Still the website has been reliable with information in the past. It is about as close to verified as you can get without a press release or post from Bay himself. Thanks to Sam N. for the heads-up.
Update: MichaelBay.com has just added a post that simply says "Welcome Nicola Peltz to Transformers 4" with a picture of the lovely new lead. I just hope she is a better ambassador for Transformers then previous leads who acted like they were embarrassed to be in the movies. At the very least she will maintain the Bay tradition of having ladies that are very easy on the eyes.