The chemical evolution of nitrogen during star and planet formation is still not fully understood. Ammonia (NH_{3}) is a key specie in the understanding of the molecular evolution in star-forming clouds and nitrogen isotope fractionation. In this paper, we present high-spatial-resolution observations of multiple emission lines of NH_{3} toward the protobinary system NGC1333 IRAS4A with the Karl G. Jansky Very Large Array. We spatially resolved the binary (hereafter, 4A1 and 4A2) and detected compact emission of NH3 transitions with high excitation energies (≳100 K) from the vicinity of the protostars, indicating the NH_{3} ice has sublimated at the inner hot region. The NH3 column density is estimated to be ∼10^{17}–10^{18} cm^{−2}. We also detected two NH_{2}D transitions, allowing us to constrain the deuterium fractionation of ammonia. The NH_{2}D/NH_{3} ratios are as high as ∼0.3–1 in both 4A1 and 4A2. From comparisons with the astrochemical models in the literature, the high NH_{2}D/NH_{3} ratios suggest that the formation of NH3 ices mainly started in the prestellar phase after the formation of bulk water ice finished, and that the primary nitrogen reservoir in the star-forming cloud could be atomic nitrogen (or N atoms) rather than nitrogen-bearing species such as N_{2} and NH_{3}. The implications on the physical properties of IRAS4A's cores are discussed as well
