eh, my .02 cents here. The sixth gear (and it's easy pull clutch) can be added to the 2013 and 2014. 2013 has same throttle-by-wire (YCC-T) engine control, traction control, D Mode engine mapping. The 2013 can even accept the 2014 and newer computer (if you really want to get CC to function over 82 Mph). 2013 does not have the ES or LED headlight/tailight, but does LED driving lights and front blinkers. The Seat, bags, windshield, two-cat exhaust (and extended list) are all the same too in this gen. Though my 2013 requires some "mods" to get closer to these later refinements, the fact that this model can easily take most of those mods, suggests there are more similarities than differences. I consider the latest models still "Gen III". I suppose an argument can be made for 3.5 refinements. However, "Gen IV", to me, would be a from-the-ground-up-rework and according to Yamaha, that is what was done starting with the 2013 model. I haven't read that proclamation from Yamaha on these latest models. Another question I'd have to ask myself regarding gen differences, are said differences, between 2013 and 2017, enough to justify a new expense? (especially if one already owns a 2013 or 2014). I'm having so much fun and enjoyment now, at this time (for me), it's hard to justify it.
Whatever the consensus, we're all family here.