I am Number 4 is an amazing book which I have just finished re-reading. The book is centred around 4, an alien from the planet Lorien, who has been living on Earth hidden for 10 years with his Cepan (guardian/trainer), Henri. 1, 2, 3, 4, 5, 6, 7, 8 and 9 and their Cepans left Lorien when it was invaded by the main protagonists the Mogadorians, in the hopes that they would repopulate the planet once the Mogadorians where defeated. After 1, 2 and 3 where murdered by the Mogadorians, 4, going by the name John Smith, and Henri flee to Paradise, Ohio. In their stay at Paradise John develops a crush on Sarah Hart, which leads to fights with Henri. Then John starts developing Legacies, powers gifted to him by Lorien. These include telekinesis and resistance to fire.
I would give this book an age rating of 11+ as there is a lot of moderate swearing. No "F" word but a few "D" words and "SH" words that parents should be forewarned about. There is also basic teenage romances. Nothing to serious but still not suitable for the young readers of 8 and under. Finally, there is some violence in fights with bullies and aliens in which John and others get injured quite badly and some of the fight scenes are relatively graphic so I would stick to 11+ readers.
All in all, this is a great book and I would rate it 10/10 for all aspects. I would really recommend after reading the book reading the rest of the series if you enjoy it.