Differentiate between a 2K and 4K video on iPad

Not easily. The viewer (most of them, if not all) will scale the video to fit the screen. Such as my 6th gen iPad, the screen resolution is 2,048 × 1,536. So both would have to scale down to fit, and as soon as you scale any image, it loses at least some detail.

Clearly I can notice the difference in detail when I change a video from 1080p to 2K right?

That's somewhat vague. I don't know if you're talking about a video online that is available to view in either resolution, or if it's something you've created. But in general, yes, either yours or my iPad will display the 4K video as a somewhat sharper image.

As I noted before, the resolution for my iPad is 2048 x 1536. Yours is 2224 x 1668. Either way, an HD 1080 video (1920 x 1080) will have to scale up to fill the screen.

As LotusPilot more fully explained, pixels have to be created from out of nowhere in order to put more pixels on the screen than exist in the source to get a 1080 video to fill the screen. This always results in a softer, pixelated image. How much the image has to scale up depends on how bad/noticeable that it gets.

4K (which is really only 2K) has to scale down to fit the screen. Pixels have to be thrown out and the rest averaged to maintain what it's supposed to look like. In short, invented pixels that don't exist in the original when scaled down. The result is a image that again ends up at least somewhat softened by the down scaling.

Neither can ever be as sharp as one to one. But, since the 4K video is much sharper to begin with than a 1080 version, even scaled down to fit, it will still be sharper than the 1080 scaled up to fit, or even displayed at actual size (not filling the screen).

Upscaling an image from a lower to higher resolution will always be visible. Whilst upscaling algorithms are getting much better, all upscaling is attempting to add image detail that doesn’t exist in the source image; there will always be image artefacts.

Downscaling a high resolution image to fit a small screen will be much less noticeable - as the smaller screen is limited to the native screen resolution. Downscaling to fit a smaller screen effective discards image detail that you’ll not be able to see. The reverse is not the case - as you are attempting to add information that doesn’t exist within the source.