4k share wont give 4k resolution on share (export)

I shot a very short (6m30s) movie in 4k. The project shows the resolution of 4096 x 2160 25p, which is the settings as per camera (Canon 5DmkID). When I finished editing I chose Apple Devices 4k, but the exported movie is only in FHD. I tried again and checked in settings and the max resolution it will give in the drop down menu is 1920 x 1080. This is only a short movie and my MBP is fairly high spec - 16Gb RAM, Radeon Pro 560 4GB, Intel HD Graphics 1536 MB. Any idea how to solve this. I have tried export to Compressor and also end up with the same result - just takes twice as long.