That spacetime position forms a four-vector (combining three spatial coordinates and one time coordinate) is a foundational definition of special relativity and standard physics, requiring no citation.
The claim that spacetime position forms a four-vector is a standard, definitional premise of special relativity (consisting of one time coordinate and three spatial coordinates). Per the system instructions, statements that are trivially true by definition or direct observation are classified as COMMON_KNOWLEDGE, requiring no papers and leaving the index arrays empty.