SO4 may refer to:
- National Identification Service, a department of the London Metropolitan Police which provides a range of support services on behalf of the Metropolitan Police and other police forces
- Star Ocean: The Last Hope, an action role-playing video game developed by Tri-Ace and Square Enix
- Sulfate, SO, in organic chemistry, a salt of sulfuric acid
- SO4 is an acronym for remembering the function of the fourth cranial nerve, the trochlear nerve, which control the superior oblique muscle of the eye.
- SO(4), the group of rotations in 4-dimensional Euclidean space
The Collaborative International Dictionary
Sulphion \Sul"phi*on\, n. [Sulpho- + ion.] (Chem.) A hypothetical radical, SO4, regarded as forming the acid or negative constituent of sulphuric acid and the sulphates in electrolytic decomposition; -- so called in accordance with the binary theory of salts. [Written also sulphione.]