The 1st 2 seasons follows the 1st 2 books pretty well
The 3rd season covers 3/5ths of the 3rd book
This season is where things start to change.
There are minor things that overlap time wise in the end of book 3 onto books 4 and 5
This is due to book 4 focusing on Southern Westeros and a little bit of Essos(Continent to the right of Westeros)
and
Book 5 focusing on the North, Iron Islands and Essos
Books 4 and 5 aren't linear,
They are parallel but in different locations if you feel me.
GRRM originally had books 4 and 5 as 1 book but decided to split it based on location
With that,
Season 4 of GoT's will focus on the remaining 2/5ths of Storm of Swords
while also venturing into a Feast of Crows and Dance with Dragons...
I'd say 1/5th of each will be in the season
So keep on reading breh
Book 3 is my favorite book...
Especially the 2nd half
Let me know if you have anymore questions