Among the Hidden -- Margaret Peterson Haddix
This is one of those books that I'd been meaning to read for (literally) years. It's the first in a series about a totalitarian society (it seemed to be set in the United States, though there wasn't a Planet of the Apes "You bastards!" moment) in which people are only allowed to have two children, the penalty being death or a bazillion dollar fine.
Luke is a third child. He knows four people. His mother, his father, and his two brothers. No one else even knows that he exists. If word got out, the Population Police would come crashing down on their heads. Life on their rural farm allows him to have some freedom--that is, until the woods around their house are pulled down to build a housing development. When the bulldozers arrive, Luke enters the house, possibly never to leave it again.
His discovery--another third child living in a nearby house--changes his life forever.
If I had read this as a fourth-grader, I would have loved it. Unfortunately, this was the first time around for me. I wasn't crazy about it. There were little annoying points that bugged me. Can't you get sick from lack of sunlight? If his parents had been really pushing the vitamin D on him, that might not have bugged me so much. But they didn't. So it did. But, like I said. As a fourth-grader, I would have been over the moon about it--and I would have wanted to read all of the sequels. (As it is, I'll probably read them anyway. As not impressed as I was, I still want to read the next one. What does that say about me?)