i don't think that we will be back to 400$ soon as price now is 650$ and we had halving 1 week ago so i am pretty sure that price won't fall down but will rise only
Compared to the $400 price, the bitcoin price will not fall down to that value. The price will be higher now.
If the price goes back to 400 dollar. A lot of miners (90%) of them will have problems to pay for their electricity. So yes it can happen, but the mining cartel will not allow the price to go this low, because their million dollar rigs will be put to waste.