I do not understand the core of your question.
If you have 4K video (3840 x 2160 @ 50 progressive frames per second AVCHD.mp4), you (manually) or the project (automatically) set the project preset to
QFHD 16 x 9
Is the video not importing when you bring it into the project using the project's Expert workspace Add Media/Files and Folders/Project Assets? Your priority in this project preset setting is the frame size unless you are going to need to do frame by frame by frame editing.
When you go to export the Timeline and if want to be as close as possible for import to export, then
XAVC-S with Presets = MP4 H.264 3840 x 2160 p25
Then under the Advanced Button of the preset, customize (for PAL frame rate 50 progressive frames per second and as required bitrate).
Please review and consider, and supply clarification and more details.