They never mentioned number of releases at all. We all posted suggesting it would likely be if they meant to include 2020.
Looking back at the slides it does kinda show up to 2020 from what's been posted and the wording wasn't clear which I feel was marketing on purpose. However yeah, no mention of 4 generations.
I've also posted an interview from the Senior Technical Marketing Manager that they meant through to 2020. Not necessarily including 2020. He even clarifies they've already met that promise with Zen 2.
Their timeline slide also clearly shows only 3 full years of support, from the beginning of 2016 through to the beginning of 2020.