If I had to make a prediction this early for 2019, I'd say:
So, the Switch wins by 8M, plus or minus a million.
Basically a reversal of 2017
More or less. I think it's reasonable to expect the Switch to be up a good amount YoY, what with Pokemon Gen VIII and the likelihood of a price cut and/or hardware revision. I also think the PS4 will be down a good amount. At this point I doubt Sony will issue any more permanent price cuts, and there's already clear evidence that the PS4 has passed its peak.