One of the most mysterious characters on Game of Thrones is Varys (Conleth Hill) and the upcoming season will be very eventful for him.
Fans of George R. R. Martin's A Song of Ice and Fire book series, on which Game of Thrones is based, already know that Varys will be an important character going forward in the series.
BOOK SPOILERS BELOW
Assuming the show writers adapt the second half of A Storm of Swords as is for season 4, Varys will turn on Tyrion Lannister (Peter Dinklage), despite being presumed close allies. Tyrion will be on trial to try and prove his innocence in poisoning King Joffrey (Jack Gleeson) and Varys will not have Tyrion's back in court.
Tyrion is then found guilty of the crime and put in prison but Varys proceeds to help Jaime (Nikolaj Coster-Waldau) in freeing him from jail. While escaping, Varys does not stop Tyrion when he proceeds to kill his [Tyrion's] father Tywin Lannister (Charles Dance). Varys then assists Tyrion in escaping King's Landing by contacting one of his friends in Essos, who awaits his arrival.
Enough evidence lines up to suggest that Varys helped Tyrion escape despite testifying against him in court, so he proceeds to plan his own escape and manages to flee King's Landing successfully. After escaping, Varys does not appear in the books for a lengthy period of time.
Varys does not get captured by the guards at King's Landing and does not appear again until the end of book 5, A Dance With Dragons.
Season 4 of Game of Thrones is set to premiere in spring 2014 on HBO though a specific date has not yet been scheduled.