NH4 is the chemical symbol for the ammonium ion. This ion has a positive charge, and is thus known as a cation. Ammonium ions can be formed by combining ammonia and acid in aqueous solution. Ammonium ions react quickly with most metals, forming salts.
The chemical formula "NH4" indicates that the ammonium ion consists of one nitrogen atom and four hyrogen atoms. Often, the formula for ammonium is expressed "NH4+" to indicate the molecule's positive charge. The ammonium ion is weakly acidic, so when it is introduced to a base, it reacts to form an uncharged ammonia ion, which consists of one nitrogen atom and three hydrogen atoms.
Ammonium ions can be generated in a laboratory setting, but they do also occur in nature. Many animals, including fish and amphibians, excrete ammonium ions as waste products in the urine. Some animals, such as humans and cats, convert ammonium to less-toxic urea before excreting it. Many plants rely on the ammonium ions excreted by animals as a source of nitrogen.
When a large quantity of ammonium ions are put under pressure, they tend to behave similarly to metals. This is thought to occur on planets such as Uranus and Neptune, and it is presumably responsible for some of these planets' magnetic properties.