if someone invested $10k into bitcoin and now it's $4k then they lost alot of money. now they are stuck in bitcoin whether they like it or not. because if they pull out now, they get to realize their loss.
If someone is experiencing something like that now, then the best option is to hold on and be patient in holding the Bitcoins that he already owns rather than incur losses that he will probably remember forever when he wants to invest again. Because he can still recover lost money through Bitcoin when there is another increase in the price of Bitcoin, it's just that he has to have a lot of patience at this point in order not to let go of the Bitcoin he already holds.