With Switch's line-up this year looking so promising for the Japanese market, I'm going to chance it and say Switch could do 4.5 million within Japan in 2019.
With such a crazy high number it would definitely need a price-cut to do it. However if it doesn't and it still manages to come even close at around 4 million, that would still be incredible for the Nintendo Switch in my opinion. Here's hoping, I think Switch deserves it.
4.5 million - with price cut
4 million - without price cut