My 4yo LOOOOVED it. There are three "Pixar cry moments" in the movie; he cried at the second one, about ten minutes before the end of the movie. It was one of his triggers, separation, and before we went into the theatre, we talked about what we would do if he got sad. He cried during "Curious George" for the same reason, so it's not like we can avoid kids' movies.

Every Pixar movie has at least one of them, so I was a bit prepared.
It is definitely heavier than I thought it would be, though. Read reviews and think about your child's temperament.