The first address they send the hacked coins to (a1) will most likely be black listed by some exchanges. However, the hackers could create thousands of new addresses and transfer the coins from a1 to the new addresses. Then do that again. All exchanges would have to monitor all addresses that a1 ever sent coins to. They could do this, but I'm not sure how many exchanges would actually do this. All it takes is one exchange to accept the hacked coins then the hacker can sell the BTC for something like ETH.
Another option would be OTC trades, but that would take a really long time to sell 4000 BTC.
The first address they send the hacked coins to (a1) will most likely be black listed by some exchanges. However, the hackers could create thousands of new addresses and transfer the coins from a1 to the new addresses. Then do that again. All exchanges would have to monitor all addresses that a1 ever sent coins to. They could do this, but I'm not sure how many exchanges would actually do this. All it takes is one exchange to accept the hacked coins then the hacker can sell the BTC for something like ETH.
Another option would be OTC trades, but that would take a really long time to sell 4000 BTC.