Why those poor guys lost $4000? Because they want to sell the coin at a lower price, if they don't sell, I won't be able to buy coins at $400. So their loss purely comes from their lacking of confidence of their investment, if they never sell any coin, they won't have any loss, and I don't need to support the exchange rate either
From the view of the central bankers, they normally don't care about the loss or profit, those are trivial things comparing to their large holding of fiat money and foreign currency reserve, the exchange rate is the only concern
But this in no case means that you're "supporting" the value of bitcoin. If fact, I would even call such stance hypocritical to a degree, since you're in the first place undermining its value by selling coins at $800 and then pretending to support it by buying it at $400 . This is plain old speculation, nothing beyond that really...
If you only bought bitcoins when the price went down (and spent it on goods or services), that would go for support
Since bitcoin's total supply is limited, if I do not create net sell pressure on the market, I would indirectly support its exchange rate
Of course I can't always sell high and buy low, I might even do it with a small loss, but the result is the exchange rate get supported and majority of the bitcoin's value kept high
If every bitcoiner realize this, that will be the biggest support behind bitcoin. Unfortunately most of the people are short sighted and only care about their own profit, so they are destined to be ruled by a few bankers who can see a larger picture