AnC60 is a metal for n=3 but an insulator for n=4, although the ratio U/W is similar, where U is the Coulomb interaction and W is the band width. The reason for this difference is partly due to the lattice structure ( Phys. Rev. Lett. 84, 1276 (2000)), A3C60 being fcc and A4C60 being bct.

Another important factor is, however, the effects of the phonons. We have compared the coupling to Ag and Hg phonons. We found that Ag phonons increase the critical value of U where the metal-insulator transition takes place, while Hg phonons reduce the critical U. We furthermore found that for the Jahn-Teller Hg phonons, the effect is stronger for n=4 than for n=3. This is a second reason why the A4C60 is an insulator but A3C60 a metal. The reasons for the difference between Ag and Hg phonons and the difference between n=3 and n=4 for Hg phonons are discussed in detail in Phys. Rev. Lett. 84, 1276 (2000)).
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