Ab initio calculations which include the effects of electron correlation indicate that Jahn-Teller unstable PLi4 has a C-2 upsilon equilibrium geometry which is only slightly lower (less than 1 kJ/mol at the PMP4/6-311+G(d)//MP2/6-311+G(d) level) in energy than the T-d structure. Comparison of the optimized structures of PLi4 and SiLi4 shows that the T-d to C-2 upsilon distortion of SiLi4 leads to a larger reduction of the Li-Li distances than the corresponding changes in PLi4. Moreover, the Li-Li distances are greater in the T-d structure of SiLi4 than in the T-d structure of PLi4 whereas the converse holds for the C-2 upsilon structures. These observations are explained in terms of strong Li-Li interactions which have been studied within the framework of the atoms-in-molecules theory. The structure of PLi4 is determined by a competition between Jahn-Teller distortions which favor a C-2 upsilon structure and Li-Li interactions and the valence-shell charge concentration around the phosphorus atom which favor the T-d structure. Li-Li interactions play a dominant role in the determination of the structure of some hyperlithiated compounds.