A theoretical framework for understanding molecular structures is crucial for the development of new technologies such as catalysts or solar cells. Apart from electronic excitations energies however, only spectroscopic properties of molecules consisting of lighter elements can be computationally described at high level of theory today, since heavy elements require a relativistic framework and most methods have only been derived in a non-relativistic one so far. Important new technologies like the above mentioned require molecules that contain heavier elements and hence there is a great need for the development of relativistic computational methods at higher level of accuracy. Here, the Second-Order-Polarization-Propagator-Approximation (SOPPA), which has proven very successful in the non-relativistic case, is adapted to a relativistic framework. The equations for SOPPA are presented in their most general form, i.e., in a non-canonical spin-orbital basis, which can be reduced to the canonical case, and the expressions needed for a relativistic four-component SOPPA are obtained. The equations are one-index transformed, giving more compact expressions that correspond to those already available for the four-component RPA. The equations are ready for implementation in a four-component quantum chemistry program, which will allow both linear response properties and excitation energies to be calculated relativistically at the SOPPA level.