Thanks all for your suggestions. I'm looking for books for DD4 to read by himself.

Wonderful suggestions everyone!

What is his approximate reading level?

I got the first boxcar children on kindle for my ds5.5 to read. He can read it but it is ds3.5 who likes it. Ds5.5 would rather struggle through zac powers or have me read famous five or the sassafras zoology book.

you could try Horrid Henry or the Secret Seven. Lots of kids seem to like Geronimo Stilton too. I liked Jennings and Darbyshire too if you can get them (they are 50's books).

I am sure someone has said this but Magic Treehouse has been a huge hit in our house. I have been reading them to her since she was 2 and she likes to act it out after I read.

I'm currently reading my 4.75 yr old Carbonel, King of the Cats, by Barbara Sleigh. I'm going to skip the last couple of chapters with the battle scene, but everything else about the book is lovely.

I believe he is reading at a 2nd/3rd grade level.

Here are some Chapter Books in the 2/3 grade level. I understand the allure of a series for gaining confidence, but there are also so many picture books in this reading level as well.

end of first grade beginning of second chapter books:

The littles (first series)
Mercy Watson series
Commander Toad series

mid second grade level chapter books:

Oliver and Amanda Pig series
Billy and Blaze series
Wishbone: The Early Years series
Cam Jansen series

end of second grade level chapter books:

Rainbow Magic series
Magic Treehouse series
Matt Christopher books

end of second grade-mid third grade chapter books:

Catwings series
Flat Stanley series
A-Z Mysteries
Beverly Clearly books
The Boxcar Children series

Most Beverly Cleary books are actually somewhat harder, like 4th/5th grade level. They're fabulous for advanced readers, though, because the content is pretty young and innocent.

My DS5 really loves the Magic Schoolbus chapter books. They aren't great literature, but they're better than Magic Treehouse, IMO (those make me want to end it all if I have to read one aloud). I think they're 2nd/3rd grade level.

You are right. the Ribsy, Henry Huggins, and Ramona books are Level O (which is a mid third grade level) but also around an 800L (which is near a fifth grade level.) Yikes, what a difference. I have only been looking at Fountas and Pinnel since we figured out that that was the system used by all the schools we are looking at.

I freakin' loved those books when I was young.