Hi Steve,
Unfortunately Ultra HD isn't supported for iClone popVideo export even with Windows 10 (I think the warning message needs updating).
However as the primary purpose of popVideo 3 is to remove green or blue screen backgrounds from standard video formats, you could export Ultra HD from iClone in WMV format, and then remove the background in popVideo 3. You will now be able to export to an Ultra HD popVideo file straight from popVideo 3.
Peter
Forum Administrator
www.reallusion.com