ashley, my kids are older and I can't think of any specific titles for you, but we used to find a lot of good non-fiction at our local used book store - my ds really liked the type of non-fiction books that he could read bits and pieces of and then go back to other parts. You could also look for a higher level science textbook to send in. Just for the heck of it, you could look into your district's upper elementary or middle school level science textbooks - you might find something he could enjoy and at the same time demonstrate he needs more challenge
And if he's 4th grade reading level and he's ready to move beyond Magic Tree House etc, has he read any of the Rick Riodan or Erin Hunter series yet? If he's not quite ready for that level, you could try Lemony Snicket series or Wayside School. I think if you get books advanced enough that the other kids wouldn't have read them yet you might get past the distracting-other-kids factor. Bones and Holes are two others that my ds liked when he was younger... but they are above the level of Magic Tree House so they might be ahead of where he's at - I'm sorry I'm not that great with levels!
Do you think the teacher would let him listen to audiobooks? Or would that be way too distracting for the other kids?
And... are there books in the school library that he's allowed to choose? Could you visit the school library with him either before or after school to help him choose a few books to check out for class?
polarbear