The thermodynamics of a superconductor, with pairing and quartet-binding BCS-type attractions VBCS and V, is investigated in the strong-coupling limit. The coupled equations for the gap parameters ΔG, Δg, corresponding to both potentials, are solved and the thermodynamic functions are computed for varying coupling constant g0 of the quartet attraction V. Some untypical thermal properties of tin, mercury, and high-Tc superconductors, which have not been fully explained by theory, are found to agree with this strong-coupling thermodynamics. Sufficiently strong V eliminates the BCS pairing potential and the conducting fermions behave as if they were interacting only via V. The structure of the condensate at T = 0 is studied and shown to consist almost exclusively of Cooper pairs if g0 ≤ 0.2G0 (G0 denoting the coupling constant of VBCS), whereas if g0 > 2G0, all fermions merge into quartets.