The Tiger Rising by Kate DiCamillo (2001)

Here's another great book by the author of Because of Winn-Dixie. In this book, DiCamillo explores the life of a boy who has experienced a great loss. He and his father are struggling through the banalities of life, and the boy is bullied constantly by the other kids at school. When Rob meets an unlikely friend, the two of them deal with the traumas they have faced together. A caged tiger brings the two of them together, and enables them to unleash their grief.

I don't know if I've ever cried so much when reading, so this might top the list. It's a lovely, well-written story with a few well-chosen words. It's easy for many kids to understand, and it is short in length. It's the perfect book for fourth through sixth grade, and it can be read quickly.