I hadn't thought about Zen growing to include 4 way SMT. If they did include that, I would hope that they make changes to the L1 and L2 caches to help with handling all of the data that would get shuffled around because of it. I would also hope that it's configurable like in the Power in that you can choose SMT2 or SMT 4, or turn it off completely. I would think that, in x86-64 world, 4 way SMT would usually not be as helpful as it is in IBM's use cases. However, if Zen 3 gets wider, it could allow greater utilization of the extra execution units.