Another alternative could be to use multi-level Otsu's thresholding to get the segmentation. You can use the function multithresh in the Image Processing Toolbox to do that.
Otsu's method and k-means clustering have equivalent objective functions (minimize within-class variance). The following paper discusses this relation:
Dongju Liu, Jian Yu, " Otsu Method and K-means ," Vol. 1, pp.344-349, Ninth International Conference on Hybrid Intelligent Systems, 2009.