CPH4 is a name used in the 2014 science fiction film "Lucy" directed by Luc Besson. In the movie, the CPH4 acts as a nootropic drug that gives the protagonist, Lucy (played by Scarlett Johansson), access to 100% of her brain capacity, subsequently developing superhuman physical and mental capabilities.
However, in reality, CPH4 is not a genuine compound or drug that can unlock unused brain capacity. The tag "CPH4" is a fictional name created for the movie. The idea that humans only use a small percentage of their brain capacity (often cited as 10%) is also a myth.
The term CPH4 does have a basis in science, though not in the way the film depicts. It's loosely based on a molecule that exists during the gestation period of a fetus. It's a substance that's produced in the body of a pregnant woman, but it does not have the exaggerated effects represented in the movie.
Learn more: