It's too early for any casting news but after watching Battleship and Brooklyn Decker I recalled an article from AICN about the possible replacements of Megan Fox in TF3 before Rosie was hired. According to AICN they had a "highly trustworthy sources" about this and if Michael and Steven really considered these actresses for TF3 then there is a big possibility to see one of them in TF4. Of course things change and in 2014 every actress from this list will have a pretty different bio and history compared to 2011, but still let's give it a try.
(click on the red names for their IMDb page)
Gemma Arterton - the first and strongest rumor. They even said that she was in direct talks with Michael about the role. This name was released before the AICN article.
Emmanuelle Chriqui - Entourage
Jessica Lowndes - 90210 (my personal favorite)
Zoe Saldana - Avatar
Hayden Penetierre - Heroes
Ashley Greenhe - Twilight
Amber Heard - Drive Angry
Julianna Hough - Burlesque
Vanessa Hudgens - Sucker Punch
Rosie Huntington-Whitely - obviously their source is legit.
Bar Rafaeli - Model
Miranda Kerr - Model
Brooklyn Decker - Battleship
If you want you can vote about your favorite from this list. And because I can only put 10 names on the list I'll get rid of some of them that I don't like.
P.S. Of course if this list was right then it was about actresses that would fit the description "Sam's girlfriend" and we don't know anything about TF4's plot so take it with a grain of salt.