'''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, {{chem|SO|4|2−}}, in chemistry, an inorganic ion or 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
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