Looks like at least three recasts for season four. The most major being Daario Naharis
Also Tommen Baratheon
(Joffrey's younger brother) and Gregor "The Mountain" Clegane
will be played by new actors as well.
This is the third actor who will have played "The Mountain" on the show. Also, the replacement actor for Tommen Baratheon actually had a different part on this past season, as Martyn Lannister, one of the two boys that Rickard Karstark killed as revenge for Jaime killing his son.