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.
I also think like that, I think after segwit there will be accumulation of prices and it would fall under $5000 or maybe it can touch $4000, but after that the price will rise very high, and this is the same as the previous segwit.
But it will recover even it falls but might take after awhile,but seeing in the past how bitoin moves then it can really still grow since btc is the future currency.