What does “4,000 watch time” on YouTube mean?

This is just math.

If you do the math, 4,000 hours is about 166.6 days. That means you have to record for more than have a year and upload it to YouTube
Let's say you have a 12 five minute videos. You need 4,000 people to watch them all of them, all the way through.

12 x 5 = 60 minutes. Times 4,000 equals 4,000 view-hours.

Yes, this is a very high threshold. But be aware, those channels that are being dropped that could not reach this number were not really making any money. 90% of the dropped channels had never been paid because Adsense requires a revenue balance of $50 to send a payment. And another 9% where making less than $100 a year.