The Switch may be stealing some short-term sales due to some persons having to make a choice which one they want first - but at the same time the reverse is also true. In the long run however, these sales are not truly lost and thus don't really affect the total sales of any console since it's just a question of when, not if. Before, this was basically a non-issue for the PS4 (it was actually more the other way around, since many couldn't get a Switch) due to the heavy stock problems of the Switch but now that the shipments got greatly expanded it starts coming into play.
The Switch stealing long-term sales (those which do affect the total sales) is also possible, especially for persons who can only afford one console per gen. But I don't think these are legion, so their effect on the total sales of both platforms should be pretty small overall.
The PS4s problem in Japan is a different one: While it has lots and lots of games, it's severely lacking in big hits, with most titles not cracking the 100k mark in Japan. While the PS4 finally managed to get a million seller in Japan with DQ XI, the Switch followed through just weeks after with Splatoon 2 and just recently even overtook the PS4 version of DQ XI in Japan, With Odyssey and possibly even BotW passing the million mark in a not so distant future. It's pretty visible on the sales front, too: Switch titles just keep selling while the PS4 titles drop off almost instantly; if the console wouldn't get a big load of new titles every week there wouldn't be many games of the PS4 on the sales charts since the grand majority of them are dropping out after just one or two weeks.