This got confirmed by SamMobile, which has a pretty good record of spot-on predictions and internal information regarding all things Samsung. As per its report, the Exynos version of the device (the GT-I9500 one) will be getting Android 5.0 Lollipop first, in early 2015. Apart from the newest iteration of Google's ubiquitous mobile OS, the Galaxy S4 will be most probably adorned with a TouchWiz that looks pretty similar to the one in the early demo of Lollipop running atop TouchWiz on the newer Samsung Galaxy S5 (which should receive the Lollipop update in December). Obviously, this is a rather substantial overhaul of the interface and most Galaxy S4 users will probably welcome it with open hands.
There is, however, no word about the Snapdragon 600 version of the Galaxy S4 (GT-I9505). We suppose that the Lollipop update for this variation of the former flagship will probably land a little after the Exynos one receives it. Are you looking forward to it, Galaxy S4 users?