We present statistical properties of 579,492 Lyman-break galaxies (LBGs) at z~4-6 over the 100 deg^2 sky identified in the wide-field Subaru/Hyper Suprime-Cam Survey. With this largest sample of high redshift galaxies, we derive the angular correlation functions with unprecedentedly high statistical accuracies at z~4-6, and identify a redshift-independent relation between the ratio of the star-formation rate to the dark matter accretion rate (SFR/dMdt) and the halo mass (Mh), which suggests the physical origin of the cosmic SFRD evolution (a.k.a. the Madau-Lilly plot). We have also conducted spectroscopy for a part of the LBGs using VLT/X-shooter and Subaru/FOCAS, and find that the neutral gas covering fraction of the luminous galaxies is >0.85, suggesting that luminous galaxies do not play a prominent role in concluding the cosmic reionization. These studies demonstrate the power of the wide-field survey to study galaxy formation and cosmic reionization at high redshift, and the upcoming Nancy Grace Roman Space Telescope will naturally extend these studies at higher redshift (z>7). We will discuss the prospects of these studies with the wide-field data obtained by the Nancy Grace Roman Space Telescope.