We have investigated several aluminum-related complexes in 4H-SiC by ab initio supercell calculations. The binding energies of the defects predict high thermal stability and complex formation between aluminum and carbon interstitials in SiC. We show that the carbon vacancy can be attached to a shallow the aluminum acceptor and form a very stable defect. We also found that aluminum interstitial forms stable and metastable complexes with one or two carbon interstitials. The possible relation of these defects to the recently found aluminum- related DLTS centers is also discussed.