It looks like someone sold a very large amount of Bitcoin in the past days, that's why the sudden crash. It's taking too long to recover because people buy it gradually. But I'm sure these buyers will not regret it if they hold long enough.
The fact that only the summer is over and autumn has begun, so this is the period when very many sell Bitcoin, because it passes the time of holidays. I've always been convinced that people sell crypto currency when they need money, and first of all for entertainment and recreation. And today it's that couple. I think that this price of $ 4,000 will not last long and we will again see a good growth in the value of the crypto currency.