Switch outselling PS4 by 6:1 without a major release this week.
What are the LTD sales for both systems in Japan? At this rate it surely can't be too long before Switch passes PS4 in total Japanese hardware sales?
Switch - 5.830m
PS4 - 7.314m
According to Media Create. Still 1.5m to go, of which a significant chunk will be made during the holiday season. Afterwards it depends on where the baselines for the two consoles settle in 2019, so giving a good estimate isn't easy. Release-wise, the first calendar quarter tends to be PlayStation's best and Nintendo's worst, so that's a factor that can push back the date.
It will definitely happen in 2019, but without knowing the margin that will be left after the holiday season, it's really hard to narrow it down to a specific month or two.