Simple scam, that's it.
NO 4k download of 4k videos. Plus, even if you want to download anything above 720p you have to pay for the premium edition of this add-on. Which then the add-on no even works within itself but redirects you to all kind of pages.
F these Scammers, making money of the work of others

Downloaded the video twice, but there is no sound!
Update 2025: does not download at all!

does not work. mp4 downloads lead to a YouTube 404 not found page, and webm downloads lead to a mysterious and suspicious blank page.

解析用网站下线，点击按钮之后无法下载

No encuentra los enlaces correctos. Y te baja la publicidad. Inútil.

Not detecting videos at all.

No longer works. Goes to a Secure Connection Failed error...

Doesn't work with membership videos.

Doesn't appear to work anymore. just opens a new blank tab...

It doesn't work at all. When im on a Youtube video and i click the button, nothing happens. (Yes, i have updated and restarted Firefox.)

It doesn't work anymore, and if it works, it downloads very slowly.

Doesn't work

Does not work at all, Click on Icon results in nothing

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.