Why aren't iStripper 4K girl cards clearly stated?

If you want to find cards that are available in 4k, simply use the "card resolution" filter and set it to 4K

Why aren't iStripper 4K girl cards clearly stated?

They are clearly marked - once you know what the indicators are.

The mechanisms (there are two of them) used to indicate that cards are available in 4k are exactly the same as those used to indicate 3k, 1080p, 720p and 480p - the card resolution and also the quality badge where Crystal Clear+ is used for 4k cards (Crystal Clear without the "+" is used for those that max out at 3k).

Using the app (I have not checked the web site) in both the store and in your own collection you can use the Quality filter (ticking Crystal Clear+) to find which cards support 4k. There is no Resolution filter in the store.

In your own collection you can use the Resolution flter to see what cards you have downloaded at 4k or, as described in the previous paragraph) you can use the Quality filter to see which cards that you could download at 4k.

To summarise
...... the Quality badge, and the corresponding filter, indicates the available resolutions.
...... the resolution in the card details, and the corresponding filter, indicates the downloaded resolution