The molecular and electronic structures of AsP3 and P-4 have been investigated. Gas-phase electron diffraction studies of AsP3 have provided r(g) bond lengths of 2.3041(12) and 2.1949(28) angstrom for the As-P interatomic distances and the P-P interatomic distances, respectively. The gas-phase electron diffraction structure of P-4 has been redetermined and provides an updated value of 2.1994(3) angstrom for the P-P interatomic distances, reconciling conflicting literature values. Gas-phase photoelectron spectroscopy provides experimental values for the energies of ionizations from the valence molecular orbitals of AsP3 and P-4 and shows that electronically AsP3 and P-4 are quite similar. Solid-state As-75 and P-31 NMR spectroscopy demonstrate the plastic nature of AsP3 and P-4 as solids, and an extreme upfield As-75 chemical shift has been confirmed for the As atom in AsP3. Finally, quantum chemical gauge-including magnetically induced current calculations show that AsP3 and P-4 can accurately be described as strongly aromatic. Together these data provide a cohesive description of the molecular and electronic properties of these two tetraatomic molecules.