What does "PO4" stand for?

PO4 stands for phosphate. It is an inorganic chemical and a salt created from phosphoric acid. A phosphate salt contains a phosphate ion, a hydrogen phosphate ion or a dihydrogen phosphate ion along with a cation.

Phosphate esters replace the hydrogen of phosphoric acid with an organic substitute, such as methyl, ethyl or phenyl. Ammonium triphosphate is the energy source for human cells, and calcium phosphate is found in bones and teeth. Phosphate is commonly found in toothpaste, baking powder and fertilizer, but it has been outlawed in detergents because it promotes the growth of algae in bodies of water.