Android 4.3 will be heading out to NTT DoCoMo's Galaxy S4 (SC-04E) in Japan from mid-December. The carrier hasn't officially confirmed the schedule, but an announcement about the Galaxy Gear mentions that the smartwatch will be compatible with the Galaxy S4 when it launches in five new colors in the country in mid-December, which is a good indication of when we can expect the update to start rolling out.
As a refresher, the Android 4.3 update will bring faster performance and improved RAM management, new lockscreen, widgets, and camera features, updated Samsung apps, new Samsung keyboard, KNOX functionality, and more. The update will be available over-the-air, and we'll be sure to let you know once it starts rolling out.