Galaxy S4 owners yearning to get a taste of KitKat can do so, thanks to an alpha build of the new CyanogenMod 11 ROM. CM 11 is based on Android 4.4 KitKat, and allows you to get a taste of Google's latest operating system months before Samsung would allow you to, officially when the update arrives in January 2014 (according to a leaked schedule memo).
If you are okay with giving up TouchWiz (many are), and can live with the many low-level bugs in this alpha ROM (should be fixed in no time) such as low mic volume, defunct screen auto-rotation, lack of video recording, etc. go right ahead. Users have reported than the magical new ART runtime is working without any hitches. Now that's good news.
CM 11 is only available for the global GT-i9500 (Exynos 5 Octa) variant of the Galaxy S4.