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.