We have investigated geometries, ionization potentials (IPs), and vertical transition energies (VTEs) of NH4(NH3)(n) (n = 0-4) cluster radicals by ab initio MO method at the correlated level. The structures in which NH4 donates as many NH bonds as possible to the hydrogen bonding with surrounding NH3 molecules are the most stable for each n. The calculated IPs agree well with experiment. The spatial expansion of the unpaired electron occurs with stepwise solvation. The growing one-center Rydberg-like nature of the cluster radicals results in the successive decrease in the transition energies to the low-lying excited states, which is responsible for the red shifts of the electronic absorption bands.