Here are the results of a Readers' Advisory question that I put forth to the American Library Services to Children Community. I had requested books at a fourth grade reading level that featured aliens.
Aliens for Breakfast by Jonathan Etra and Stephanie Spinner
Spaceheadz by Jon Scieszka
Lunchbox and the Aliens by Bryan W. Fields
My Teacher is an Alien by Bruce Coville
Space Race by Sylvia Waugh
STAR HATCHLING by Bedard
The Outernet series by Steve Barlow
The "Alien Agent" series by Pamela Service
"Sleepers Wake" by Paul Samuel Jacobs
The "Sardine" series of graphic novels by Guibert & Sfar
The "Jellaby" series
"Only Child" by H M Hoover
"Away is a Strange Place to Be" by Hoover
"This Time of Darkness" by Hoover
"Alien Secrets" Annette Curtis Klause
Flight of the Outcast by Brad Strickland.
"Adventures of Commander Zack Proton"
Daniel Pinkwater's FAT MEN FROM SPACE?
Stepping Stone books by Jonathan Etra
The True Meaning of Smekday, by Adam Rex
Stephanie Spinner's Alien Series
Boom! by Mark Haddon
Attack of the Fluffy Bunnies by Andrea Beaty
The Fallen Spaceman by Lee Kingman