ABC mysteries, Calendar mysteries by Ron Roy. DS loved them at 4-5 and we loved them because there was no friend drama whatsoever!!! They liked each other, their parents were nice. No orphans! But they had a neat little mystery to it.

Frog and Toad
Commander toad series by Jane yolen - hilarious rip off of Star Trek
Magic school bus chapter books - the science and language are actually easier than the original picture books - which I highly recommend too
Mammoth academy series by Neil Layton