The Fourth "R" (also known as The Brain Machine) is a science fiction novel by American writer George O. Smith, first published in 1959.

It was reprinted again by Dell under its original title, The Fourth "R", in 1979 (paperback #13419, ISBN 0-440-13419-6) At the beginning of the story, Jimmy's mother and father are murdered by their best friend, who is also the youngster's godfather and appointed guardian as well as the inventors' trustee.

Jimmy must survive his guardian's efforts to squeeze the secret of the invention out of him (whereupon his death will most certainly be arranged, just as his parents' were), and then escape into hiding until he can grow into a physical stature commensurate with his mental age.

In the process, the character must make for himself a living and a safe place of residence, and Smith uses his protagonist's situation and capabilities to examine the nature of childhood and the "protections" (including incapacitations) imposed upon legal infants in American civil society at the time of writing.

[1] R. D. Mullen reported that "Though it becomes tendentious and sentimental in its last chapters, [The Brain Machine] is up to that point a surprisingly good story of the difficulties of the superboy in a world run by stupid adults.