Current 24-hour volume is 520,641 BTC according to Coinmarketcap. Even if you sell those 4k BTC on one exchange, I doubt we'll see much movement either way.
Kobayashi might say otherwise....
How much of that volume is fakes so that some Chinese exchanges can claim to be nr1 in the world?
One 1000 BTC drop and we went down 300$ last time, I'm not that eager to see 4000BTC dumped at once.
But the feds will never sell the coins on an exchange with fluctuating prices.
It wouldn't be the end of the world, but at current prices it would be a problem, given the fact that we seem to be stuck in nowhere's land on this sort of $6k limbo. If someone dumps 4000 at once, we could lose the $5000-ish resistance which would lead to further panic selling. If the 4000 sale happens on other price ranges, it wouldn't matter that much.
Governments are supposed to use OTC market to auction the coins, but Kobayashi signed up to Kraken to dump the Gox stacks. Govs can be incompetent and screw up Bitcoin investors, they can be reckless. If they were smart they would be buying the coins themselves or simply holding them in vaults. Imagine if they held all the Silk Road coins.