Which (exact) version of InDesign?
We think we have linked the issue to the secondary screen attached to the iMac. We'll do some more testing but wanted to find out if people were using the iMac 4K with the Creative Cloud suite just fine.
Do a search on this forum – I think the matter has been discussed by others.
4k iMac do not exist, I guess you mean 5k ;-)
What GPU are does your iMac has? Is it Intel by any chance?
If yes then this should help