We have determined why hydrothermal synthesis does not turn out to be an adequate method to obtain nanoparticles in the Sr-Mn-O system. At high KOH concentration, the appropriate one to obtain nanoparticles, Sr is partially incorporated inside an intermediate (K, Sr)xMnO2•nH2O layered phase that prevents the 4H-nanoparticles formation. Numerous experiments have been performed by screening the parameters involved in such process (precursors, pH, temperature, etc...) in order to understand their influence in the shape, size and purity of the 4H-final product. Analysing the experimental results by means of XRD, TEM, TGA and EDS among others, some peculiarities of the synthetic procedure have been detected; the appearance of side phases, SrMn3O6-x nanoribbons or the above mentioned birnessite-type (K, Sr)xMnO2•nH2O phase, and the insertion of K within the 4H-phase prepared at high KOH concentration. Magnetic features are also discussed.