The insulating mixed-valent Ir+3.66 compound Ba4NbIr3O12 hosts two holes per Ir3O12 trimer unit. We address the electronic structure via resonant inelastic x-ray scattering (RIXS) at the Ir L3 edge and exact diagonalization. The holes occupy quasimolecular orbitals that are delocalized over a trimer. This gives rise to a rich intra-t2g excitation spectrum that extends from 0.5 eV to energies larger than 2 eV. Furthermore, it yields a strong modulation of the RIXS intensity as a function of the transferred momentum q. A clear fingerprint of the quasimolecular trimer character is the observation of two modulation periods, 2π/d and 2π/2d, where d and 2d denote the intratrimer Ir-Ir distances. We discuss how the specific modulation reflects the character of the wave function of an excited state. Our quantitative analysis shows that spin-orbit coupling λ of about 0.4 eV is decisive for the character of the electronic states, despite a large hopping ta1g of about 0.8 eV. The ground state of a single trimer is described very well by both holes occupying the bonding j=12 orbital, forming a vanishing quasimolecular moment with J=0.