For the record I was finally able to create an acceptable .MOV H.264 video.
Things to note.
1. 4K has to be 3840x2160. Video shot in 4096x2160 has to be rescaled to 3840x2160.
2. Export the video from Premiere Pro CC 2018 as a .mp4 encoded with H.264
3. VideoProc by Winxdvd can then export your mp4 as .MOV and encode with H.264. This step seems to be a waste of time but Getty wants .MOV 

Now the hardest decision to make is whether I could be bothered seeing as nearly everybody else takes 4096x2160 H.264 MP4s as export from PP CC 2018. 

As far as the suggestions to use DaVinci or PP CC 2017 go.   
Thanks but we have what we have but you if you want to go to the extra expense or effort go for it.

We tried using the recommended export settings suggested by iStock however it doesn't seem possible using Premier Pro CC 2018 on a Windows machine.
Not for 4K or HD. It isn't possible to encode h264 in a MOV container any more with PP (apparently).
If anyone has any suggestions on what settings we can try we'd appreciate it.
Storyblocks wanted .MOV for HD footage and we encoded it using DNxHR and it was accepted. No such luck for iStock.