The first 4 seasons cover only the first 3 books actually.
And it's note like they didn't pick and choose and leave out a ton of stuff and characters. I have no doubt that what he dumped on them was legit five more seasons worth of things to see and do. The problem is when they made the choices to leave out or drastically alter the scenarios or characters used they end up with their back against the wall. Now maybe one could argue that those choices were neccesary for the flow off the show, maybe book Euron wouldn't work or maybe they didn't like the detours for instance. But the choices were made by the showrunners so they dug their own hole, and can at this point not yell up from their selfdug hole that hey George you didn't tell us how to get out of here. You were in charge, it was on you to bring it home and you were found lacking