I have AVG antivirus installed on my computer and I do not know if it is due to AVG but I could not download the 4K file at https://www.youtube.com/watch?v=LarDrhbO7b0
even after allowing the download to proceed.

Here is what AVG says:
Threat secured
We've safely aborted connection on s5.youtube4kdownloader.com because it was infected with URL:Malvertising

I am very disappointed!

To answer the developer:
The big problem is that, with AVG installed, I cannot even reach the stage where I can download a video file through this addon because AVG does not allow me to do so and AVG does not even allow me to override their blockage of the downloaded file in any way.