4k textures will get downsampled to 2k when importing to UE4. As for quality differences between viewport and play mode, that is interesting. Perhaps there is a quality setting being set between the two.
when i understand it right, that is basicly the problem with the substance-4-k-support in ue, which should be fixed in the next months (info from allegorithmic)…
after all, when this problem is fixed, i would be able to see real 4-k-textures in the game mode?