I think it's a tricky stage to find books for. DS2 is 5 in May, and in the past year things that have gone down particularly well (in addition to the picture books) along these lines (longer books/fewer pictures) are:
Dick King-Smith: Sophie's Snail, Dragon Boy, and The Cuckoo Child
Andrew Wakefield: Bottersnikes & Gumbles
Michael Bond: A bear called Paddington
I can't remember the author: Milly Molly Mandy
Dorothy Edwards: My naughty little sister
Enid Blyton: The Magical Faraway Tree
Michael Bond: A bear called Paddington
Roald Dahl: Fantastic Mr Fox, The giraffe et al.
Also any 'easy readers' from the library, or bookclubs, tend to have simple plot lines, and short chapters, and it will be a good while before he can read them himself.
Just to add I think it's an excellent thing to do to get 4 yr-olds to listen to longer stories, and concentrate on words not just pictures, and to expand their vocabulary if they are ready for it.
I remember being slightly discouraged by some of the response (don't do it - stick to picture books) to a similar question on mumsnet over 2 yrs ago about my (then) 4 yr-old ... Well he's now 6.5 and is a superb reader, and confidently tackles 300-700 pp books, such as The Hobbit, and Harry Potter 4 & 5.
So I would say "go for it" jimmychoos IMO it's the most important thing you can do for his literacy at this age, and you are giving him a precious gift.