If all you are after is watch time to build to 4k hours, then just run a TrueView campaign in your ads account. Those are the videos that play at the start of other videos. Make a 4 hour video and you'd be surprised at how fast watch-time will build up, and it won't cost you much.
The caveat is you won't get any engagement on the video itself, to get engagement you need to run Discovery ads. But if it's just watch-time you need, stick with TrueView.