I know you know this, but every kid is different. And it'll work itself out.
My PG son started reading books fluently when he was 3 1/2 and was devouring chapter books by K. He loved fiction, non-fiction, magazines, you name it. And he still does.
My dd started reading around 3 1/2, too but never really wanted to read a book on her own. I bribed her the summer before K to read one of the stink books and she read it in a few hours, but she wouldn't read again for me on her own and refused to participate in any kind of bribery.
When she was first, she finally had a teacher who knew how advanced she was, and she assigned some incredible Newberry-winning books in 1st grade. She was required to read them and write a response. And by December of her 1st grade year, she'd read 6 long chapter books in class. She's in 2nd grade now and she just brought home her reading test and it's at 8th grade level. But she still rarely reads at home. We read to her every night and if we're too slow getting to her room, she'll go ahead and read, but otherwise she just waits for us.
If I were you, I wouldn't worry too much about it.
