The Special Theory of Relativity encourages one to think of space and time as being different labels for coordinates in a single entity called spacetime. As such, instead of using x (three dimensional) and t (one dimensional) in equations, treat the two together as a single object x^μ=(t,x)^T (four dimensional): this is called a four-vector.
Four vectors extend to covering other things besides space and time: they can represent any quantity which transforms in the correct way between different frames of referrence (ie. by Lorentz Transformations: x' ^μ=Λ^μ_νx^ν). Such possible quantities arise from pairs of (three) vectorial and scalar quantities, eg:

• energy (E) and momentum (p)
• electrical scalar potential (φ) and the magnetic vector potential (A)
• fluid density (ρ) and flux (f)

Four-vectors and their associated higher order tensors are conventionally written down using the abstract index notation.