Hi Tomas J:
If you can edit 4K in iMovie then you can do the edit and transfer to project to FCPX for a 4K output if you're limited. but either way you need to get FCPX for output.
I still think XAVC-S is not fully supported by FCPX many problems encountered with the Import window.
Ingesting clips into FCPX does not mean that it support it, as this has to do with recognizing the file structure for the codec in the SD, card.
In my case importing from the FCPX import window if I have all 4K XAVC-S clips, FCPX will see the SD card as a Volume and not as a Camera but if I add a reg Mp4 clip or AVCHD clip to the rest of the 4K clips on the same card it will recognize it as a Camera the way it supposed to work. Go figure Why???? I wonder how you have been doing it since you have the AX-100 which the same file structure than the AX-33 that I have.
Thanks
AppleLou