Most anime made in digital are often done in lower resolutions then upscaled. More pixels just becomes more work and at a certain point it becomes very inefficient and expensive. IIRC most anime are done under 1080p then upscaled to 1080p during the compositing stage (or maybe even after compositing?).

The only anime that could be "properly" upscaled to 4k are those that were done traditionally in cel, provided that it's physically big enough. The same is true for film.