Despite escalating this issue to Adobe the problem of uploading larger files for me is still not resolved.
Regarding the comment from Kglad, it is correct that any file of 4.3GB will take a long time to upload. This has always been the case, it will also depend of course on your internet upload bit rate as well as Creative Cloud's ability to handle the file.
The behaviour decribed - that Creative Cloud reports a message 'Unable to Sync' immediately is not the way this process has always worked in my experience.
Previously, on dropping a file (even a large file - less than 5GB) into a Creative Cloud folder on your desktop - the file would copy across locally and immediately start the sync process with the upload. There is no message to say 'Unable to Sync'.
You could monitor status through the Cloud Icon (just says Syncing).
Then, once all files are sync'd (uploaded successfully) the status on the cloud icon would be say All Files Sync'd.
The current issue I am facing suggests there are problems with Creative Cloud Server functionality. That is what Adobe support are supposed to be investigating, since the video file I am uploading meets all necessary criteria (eg size/file name/etc)
When I have some answers from Support on the problem I am facing I will post here which might help other people.
Finally - I would add that using Microsoft One Drive to upload a file of over 4GB is much faster than Creative Cloud, and for the moment I am using that method to distribute the completed Video File.
Unfortunately you need to have another subscription to do that of course.
All input on this to help resolve it is appreciated.