No announcements have been made, so no one can tell you when it will come to market. PCIe 4.0 may not come in 2019, if it does it will be limited. One of the issues is it requires a motherboard that supports PCIe 4.0 and another component. So if AMD or Nvidia release a GPU that runs on PCIe 4.0, a chipset and motherboard would also have to be released that can run on PCIe 4.0.
My guess, that if PCIe happens in 2019 it will be when Zen 2 is released for Ryzen or Threadripper parts (1st half of 2019. Or when Intel's 10nm parts are released (Q4 2019).
But honestly, I dont think you will see anything till 2020.