Any 4k you got during your Premium will remain yours. Only a refund would justify that we remove them :)
To clarify, a 4k card purchased during Premium would remain both downloadable and playable at 4k after cancelling Premium, or would only remain playable at 4k but not downloadable at 4k?
In other words, if a 4k card had to be re-downloaded later for whatever reason without Premium, the 4k version would remain available to download?