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
• The Last Hope, an action role-playing video game developed by Tri-Ace and Square Enix
• Sulfate, SO₄²⁻, 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

• Austria – Italy relations
• Austria – Kosovo relations
• Austria – Pakistan relations
• Austria–Philippines relations
• Austria – Republic of Macedonia relations
• Austria – Saudi Arabia relations
• Austria – South Korea relations
• Austria–Switzerland border
• Austria–Turkmenistan relations
• Austria – Ukraine relations
• Austria – U.K. relations
• Austria – U. K. relations
• Austria–UK relations
• Austria–U.K. relations
• Austria – UK relations
• Austria–U. K. relations
• Austria – United States relations
• Austria–Uruguay relations
• Austria–US relations
• Austria–U.S. relations
• Austria – U.S. relations
• Austria – US relations
• Austria – Vietnam relations
• Austric race
• Austrijsko primorje