Effects of electron correlation, intrasite vibronic interaction, and merohedral disorder on the electronic structure of K4C60 are investigated with a model approach taking into account all essential interactions in the lowest unoccupied molecular orbital (LUMO) band. The self-energy was calculated within the GW approximation with self-consistency after the quasiparticle Green function starting from Hartree-Fock band structure. The insulating state arises due to interorbital charge disproportionation within the LUMO band while the band gap is strongly reduced by effects of long-range electron correlation. The results of the calculations are in reasonable agreement with experiment, providing evidence for a Jahn-Teller induced transition from a Mott-Hubbard to a band insulator state. [S0163-1829(99)51020-5].