After their planet Lorien is destroyed by the monstrous Mogadorians, 9 children came to hide on Earth. They can only be killed in numerical order, so when Number Four realizes that the first 3 have been slain, him and his protector flee for safety while searching for answers.
The movie variant cover of the book I Am Number Four is based on(also called I Am Number Four) contained a quote from Michael Bay saying that the protagonist was a "hero for this generation." The cover didn't mention Bay had produced the movie.
There are nine of us who escaped. Number one was killed in Malaysia. Number two was murdered in England. Number three was hunted down in Kenya. Before they come for the others, they will come for me. I am number four.