People are seriously making that claim. Strong possibility they are trolling but if trolling makes you look silly then jokes on them.

Switch is cannibalizing Wii U sales in Japan

Cannibalize - When a company reduces the sales of one of its products by introducing another similar product.
So, if the Switch were cannibalizing the sales of any product, it would be the 3DS and/or Wii U, since both systems are made by Nintendo.
So, "cannibalize" isn't the right word here.

As far as the PS4 goes, with or without the Switch, it would still be selling the numbers that it is selling because the dedicated console market in Japan is in decline, it's been in decline for a long time now.

Japanese Home Console Sales:
PS1-N64-Saturn: 30.7 million
PS2-GC-XB-DC: 30 million
PS3-360-Wii: 24.9 million
PS4-XB1-Wii U: 8.86 million (thus far)

Based on where the dedicated home console market in Japan is today compared to 10-15 years ago, the ceiling has gotten considerably lower for the PS4.
Best case scenario I see is that the PS4 sells 15 million in Japan, but that would still put it well below the PS1 and especially the PS2. And the PS4 is the current leader in the dedicated home console market. It's no fault of Sony's, the market is just not there the way it used to.

As for the Switch, it was obvious from its reveal that the Switch would take over Japan and become THE new king of that market and no amount of Monster Hunter for PS4 can change that outcome. Even the most skeptic and pessimistic doubters of the Switch at the time of its reveal said it would at least do well in Japan.

So, long story short. The sales of the Switch and PS4 have nothing or at least very little to do with each other.
One is selling in a market that is on the decline. The other is selling because it is serving as a bridge between that market and the new dominant, portable market, and quite simply, it's the next Nintendo portable.

I don't think it's right to say that people are using that as the only reason for the PS4 being down YoY. There's many reasons.

To argue against one point on your OP though... Switch wasn't eating into PS4 sales throughout the year because there was no Switch stock to do so. Right now, we are actually seeing what the machine can do when there is stock available.

Very well said.

wow lmao

Primary the Wiiu sales, then PS vita SAles, in 2018 3DS remains sales.

The switch is the sucessor of the 3DS, Wiiu and Vita.

You do realize cannibals eat their own right?

yeah, all are videogames, right?

Same species :)

It's no cannibalism if they're products in competition with each other.