What does NO4 stand for in chemistry?

'NO' is not an element, and is therefore not found on the Periodic Table. The chemical NO is nitric oxide (aka nitrogen(II) oxide), a colorless gas at room temperature.

If, however, you are referring to 'No', then it is the symbol for Nobelium, which is a synthetic, metallic, radioactive element with atomic number 102. It can be found in the actinide series.