From phenomenological considerations, a mass formula is derived to calculate the interaction energy among the last two neutrons and last two protons in a nucleus. This interaction energy is the intra-α-cluster energy of the last α cluster in the nucleus. Then from a proper analysis of these intra-α-cluster energies, a separation of the intra- and inter-α-cluster energies out of the total binding energy of the nucleus is made. Clear ideas about the sizes of the α clusters relative to the size of the free α particle and also about the degree of α clustering in each α nucleus are obtained. Positive evidence supporting the additive nature of the α-α-cluster interaction is found. Finally, the intra-α-cluster interaction energies are compared with the nn-, pp-, and np-interaction energies in the same even-even self-conjugate nuclei in order to explore the similarities between the nature of two-body and four-body interactions. © 1972 The American Physical Society.