Four days for each book? Wow, that's pretty quick!
I remember reading through books 2,3, and 4 fairly quickly (I can't remember in how many days) although I slowed down on the later books. I'm not entirely sure why that was - perhaps it was all the POV switching. When I do my next reread, perhaps if/when the last book comes out, I'll read them much slower too.
Once you get into them, though, they really do draw the reader in, in my opinion. Especially the early books.