Clipgrab can only support to download up to 1440p videos but I hope it can download 4k quality as I have a 4k iMac and some videos are available as 4k quality.
try setting the "Use WebM if possible" option in Settings->Other.
With 4k monitors so cheap now, it would be nice to grab 4k in 4k.
I vote for that feature!
please add 4k support