Smart TV or Fire TV 4K?

I have now bought a 4K Smart TV that can do netflix and Prime, but I'm also advised to get a Fire TV 4K stick, but why?

Does this mean that my 4K Smart TV can't access 4K content or is this only possible to a limited extent?

You only need the stick if you don't have a Smart TV.

4k is not worth it anyway, because very few films and series are available in Netflix, Prime etc. In 4k.

Depends on the TV, but my experience is that the software of the Smart TVs is much slower, unstable and jerky than e.g. With a Fire TV Stick. Therefore, I would always recommend a Fire TV, Fire TV Stick or Apple TV.

However, ard and Zdf media library use 4K for some unstable things and since the film industry is technically better, I thought, it can't hurt.
I only heard that the Smart TV function is not worth it and you should better take a Fire TV Stick

But before the industry has upgraded, it will take years. Until then, you buy yourself a new TV.

So I have a Samsung Smart TV and everything runs perfectly and easily. But as with other models, I don't know.