We present the results of a search for the Diffuse Supernova Neutrino Background (DSNB) at Super-Kamiokande (SK) that incorporates $22.5\times2970$~kton.days of data from its fourth data-taking phase. Two analyses, with different energy regimes covering the 9.3$-$81.3~MeV antineutrino energy range, have been developed based on previous SK searches, with substantially improved background modeling and data reduction techniques. Moreover, a combined analysis of nearly 20 years of SK data (1996-2018) has been performed, with a 90\% C.L. sensitivity to the DSNB flux comparable to various DSNB predictions. No significant evidence for a DSNB signal has been observed, and both analyses set world-leading limits on the DSNB flux that can reach about $2.7\overline{\nu}_e$.cm$^{-2}$.s$^{-1}$ for a wide range of models.