Yes $4K is the perfect time to buy bitcoin. The problem is when, we don't know it happens. I think it's a loss have to wait while we want to invest in bitcoin.
If you have a plan to inverse means, just wait to reach 4000$.It occur only once I think. After the Segwit 2x,the price of bitcoin will reach .So wait till the end of October and brought the enormous bitcoin at that time. And don't sell it soon.Sell after the one month and you will get very huge profit.
But i think there is no use to wait, because now its look like difficult that we will see bitcoin price again trading in such a low price. Bitcoin price is increasing from time to time and there are more chances are that bitcoin price will very soon cover 8000$, therefore i do not think that it will be a wise decision to wait for the price to decrease but should try to invest in current price and get some position.