Infrared predissociation spectroscopy of vacuum ultraviolet-pumped ion (IRPDS-VUV-PI) is performed on ammonia cluster cations (NH₃) (n=2-4) that are produced by VUV photoionization in supersonic jets. The structures of (NH₃)₂⁺ and (NH₃)₄⁺ are determined through the observation of infrared spectra and vibrational calculations based on ab initio calculations at the MP2/6-31G** and 6-31++G** levels. (NH₃)₂⁺ is found to be of the "hydrogen-transferred" form having the (H₃N⁺-H···NH₂) composition. In contrast, (NH₃)₄⁺ exhibits the "head-to-head" dimer cation (H₃N···NH₃)⁺ core structure, where the positive charge is shared between two ammonia molecules in the core, and two other molecules are hydrogen bonded onto the core. An unequivocal assignment of the infrared spectrum of (NH₃)₃⁺ has not been achieved, because the presence of two isomeric structures could be suggested by the observed spectrum and theoretical calculations.