You can buy if you want now, but i would advise you to wait for bitcoin to fall even harder, because you can make more money that way. I think that bitcoin will fall very hard and that one of the altcoins will rise up and take over the place of bitcoin. So you can also invest in some altcoins. Lite coin for an example is the sleeping giant.
I do not think bitcoin will fall even harder. It has always been this way. As a matter of fact, bitcoin is decreasing and I know it had a sharp fall, but it will not go down any longer. Reaching 4000 was a great achievement for bitcoin to be honest. People should start to buy bitcoin for now because I believe it will rise again, and when it rises, it will prosper.