Well it's your own choice to buy one whole bitcoin but if you are waiting for the price to go down as what you mentioned so that you can afford it then that time may not come. The last dip around $6,000 maybe the lowest price you could have bought it ever. If ever the price would reach $4,000 then there are still so many people who cannot afford it or those who can afford but are not willing to buy it at that price because they are waiting for it to go lower in which case they miss the chance to buy at a lower price. If you wanted to invest in bitcoin for the long term then you should just buy what you can afford whatever its price and it doesn't matter how many it is as long as you are willing to let go of the money in case something unfavorable happens.