Assuming it is made by TSMC and Nvidia, and it needs to be 6 times faster than the Switch, then:
10 nm TSMC = 2x shrink, available this year already
7 nm TSMC = 2.8x shrink, 2019-2020
5 nm TSMC = 4x shrink, maybe 2021?
3.5 nm TSMC = 8x shrink, maybe 2023-2024, coincidentally also TSMC's last planned node shrink.
Add one or two years since brand-new nodes are rarely used for consoles, but accounting for more efficient GPU architectures, I'd say between 5-8 years. Let the too optimistic be reminded, of course, that the Tegra X1 once had the hype it could maybe match current consoles, and look how it turned out.
Edit - added TSMC panel from the ISS conference as evidence for any of those not used to seeing these nodes and what they really mean. Note that the increased wafer cost means every mm2 of the chip (even though transistors / mm2 still decrease in price) will be more expensive that it is nowadays, further limiting how soon the "PS4 Switch" could launch:
