At 4 my DD's favorites by far were the Berenstein Bears. She still likes to pull them out occasionally and re-read them now at 8.

Just be aware that there's a small subset of those books that carry a fundamentalist Christian message, and are unidentifiable from the rest except for a tiny "Living Lights" icon and Mike Berenstain's name (Stan and Jan are the original authors) on the front cover. So unless fundamentalism is your thing, you'll want to avoid those.