The 4(I)Sigma(+) state of NaK molecule has been observed for the first time and studied by polarisation labelling spectroscopy technique. A total of 1567 rovibronic levels in this state were measured with an accuracy better than 0.1 cm(-1). The inverted perturbation approach method was used to construct the potential energy curve of the 4(I)Sigma(+) state. The irregular shape of the potential confirms earlier theoretical predictions. (C) 2003 Elsevier Science B.V. All rights reserved.