I think it will not have that much of an impact for at least the coming three years. As for this moment third party games tend to sell rather well on the PS4. Especially western third party games do sell very well on the PS4. While it's possible to make PS4 ports run on the Switch with less effort than making PS3 ports run on the Wii, I doubt many companies are going to abandon the PS5 for the Switch. The Wii had the position that it's userbase was bigger than the combines userbase of PS360, yet even that wasn't enough. In the end Sony has very good relationships with devs Nintendo just doesn't have. If the PS5 is going to be the 8-10TFLOP console with 12GB RAM and a good CPU than I would doubt if porting PS5 games to Switch is going to be easy, especially complex open world RPG's. In the end more Japanese developers will support the Switch and more Western devs will support the PS5 and X2. As for Sony being pushed out of Japan. It is still selling PS3 like levels. So sure it's going to be outsold by the Switch, that's written in stone, but there is still a market for PS4 games in Japan. Whether there will be a market for PS5 games is a relevant question though in Japan.

In short more Japanese devs for Switch, but westerns devs will stick with Sony is my prediction, except for second party exclusives obviously.

The Switch will be a success everywhere, and japanese companies will support it more and more, specially with their titles focused on japanese tastes. However, I don't see why that would affect negatively to the PS4. I'm sure porting from the Switch to the PS4 is easy, so even if the Swich became the main japanese platform for developement, the PS4 would still get games. And also PC, which is good.