The properties of the Si(C)-C(Si) antisite pair in 4H-SiC as well as its formation mechanism have been investigated. For the formation of an antisite pair in the ideal bulk a high energy barrier has: to be overcome. However, in the presence of other defects the formation energy and the barrier are significantly lowered. Here, we present our results concerning formation energies and diffusion barriers for the formation of an antisite pair nest to silicon- and carbon-vacancies as well as to the V(C)+C(Si) pair which arises from V(Si). Structure and formation energy of the antisite pair have been calculated within ab initio density functional theory (DFT) in the local density approximation (LDA). These results are well reproduced by a charge selfconsistent DFT based tight binding method (SCC-DFTB), which was used to calculate the diffusion paths and the activation energies for formation.