In this article, we investigate the four-level random telegraph noise (RTN) characteristics of a gate-all-around (GAA) nanowire (NW) transistor. The RTN-testing devices were fabricated with the sidewall spacer etching technique. The effective channel length and width are approximately 150 and 30 nm, respectively. By decoupling the four-level RTN, we are able to extract the time constants associated with the two traps. Circle-shaped approximations are used to mimic the triangular NW for evaluating the depths of the traps. The extracted depths of the two traps are very close to each other, which is consistent with the time-evolution measured results. We've also explored the probabilities of transitions between two specific current levels in the RTN characteristics, as well as the relative trapping/de-trapping frequencies.