Tetra-atomic BC2N, the building block of the mixed carbon and BN clusters, has been studied at the HF, MP2, and CCSD(T) levels using both double- and triple-zeta basis sets with polarization and diffuse functions. In contrast to the parents C-4 and (BN)(2), the linear triplet ((3)Pi) BCCN is found to be the most stable and the linear-cyclic energy difference is about 28 kcal/mol. In the cyclic structure, isomers with adjacent B and N atoms are more stable, whereas no general trend of atomic combination has been found in the linear isomers. The preferred sequence of atoms in the linear form depends on the cluster size. The nature of bonding and atomization energies of the parent and hybrid molecules are compared and discussed.