I would second all of the choices on here (although I have bypassed the Boxcar Children for my newly 4-year-old). We've also had success with:

Cam Jansen
Nate the Great

Authors we love:
Arnold Lobel
Cynthia Rylant
Doug Cushman (Aunt Eater Loves a Mystery and others)
Kevin Henkes (not chapter books but nice stories and pictures)

As a space fanatic, he loves the Commander Toad series.

We've just started getting into Magic Treehouse, which I was concerned would be too scary. However, he's loved the three that we've read. Likewise, we've also just started reading the A-Z mysteries, which again have received a good reception (and are not too scary, but usually involve robberies - so I've had to explain the legal system to him, which we now reenact with Legos).

I like the suggestion of picture books. I haven't had much luck with our librarians. Can anyone suggest good picture books for 4-year-old readers, too?