Do 4k-Premium Clips bring better Quality to lower Resolutions?
I use a 2560x1440 Screen without Premium, so using the 3k-Shows with round about 80 % Size in mini-Modus.

Now i am thinking about getting Premium and dont know, if there well be a quality-Boost with no 4k-Screen. Any experiences with Quality-Changes on my Resolution?
Unfortunately there is still no possibility to get maybe 1 4K-Demo to check out the Diference, right?

You definitely can see difference between 3k and 4k clips on 4k monitor. But I tested them on 1366x768 screen and haven't noticed any changes even between 1080p and 4K clips. So, I suggest you to buy 4K monitor for 4K clips otherwise you just going to use 1,8Gb of free space per card instead of 1.0Gb.

Now i am thinking about getting Premium and dont know, if there well be a quality-Boost with no 4k-Screen. Any experiences with Quality-Changes on my Resolution?
Unfortunately there is still no possibility to get maybe 1 4K-Demo to check out the Diference, right?

As @shadowgamer says, there probably won't be a major difference between 3k and 4k on your monitor. If you do decide to upgrade the monitor to 4k, bear in mind that the cpu load is significant. You may need a bigger cpu, to cope with decoding and scaling the 4k video, or the show will stutter - motion will be jerky. If the cpu load is more than about 25% with 3k shows, then in my experience 4k may be a problem.

if there well be a quality-Boost with no 4k-Screen.

For my eyes, even with no-4K screen there is quite a big difference from a resolution to another one - and 4k one is far better than 3K, of course.
Everyone said to me I should use only 1080p or even 720p animations, but personally at 1920x1080, and even at 1366x768, I still see quite a big difference in image sharpness and details on shows - not only if going from 1080p to 3K, but I see same big difference from 3K to 4K also.
Anyway, I know that many others see at my screen resolution(s) absolutely no difference using different resolutions of iStripper clips - so everyone should probably rely to his own eyes only...