The problem seems to be solved!
The Attenuation Radius of one of my house lights was ridiculously high, basically including the whole scene and lighting the whole scene up, erasing any shadows in the trees.
However I still have no idea why this effect only kicked in in higher render resolutions but not during renderings in HD and why it seemed to only effect my foliage but not the rocks and cliffs.
Nevertheless, after scaling down my attenuation radius, everything seems to be rendering fine now also in 4k.
Thanks to everyone trying to help!