Unless I could listen to the speakers in person, I wouldn't consider sending a cashier's check to somebody prior to having the speakers in my possession. And, $4K isn't something that would warrant such demands.
From my days of buying/selling watches, I have seen numerous posts about cashier's checks being bounced, as well as wire transfers.
Unfortunately, we live in a world where a few bad actors spoil online buying, so I guess the seller is trying to protect himself.
One possible course of action is to ask if he has an Ebay, USAM, or Audiogon account, and you would be able to see if he has positive feedback.
B