How do I pay?

PayPal is not always protection.

IF there is an invoice for goods (i.e. specifically naming the speakers), then you have PayPal protection.

IF you just pay via PayPal, without an invoice, it is 'sending money to friends or family', with ZERO protection.
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Seller pays a fee to PayPal when selling goods. You can offer to pay the $4,000. plus the fee via invoice thru PayPal.

If seller still refuses, I would walk away.

Pass on the sale, other opportunities will come along.

USAM has a "Payment Method" line item on their posted sales listings.  It is the seller's choice how they want to receive their payment.  If a cashier's check was the payment method requested by the seller, then you are welcome to ask whether they will accept another method, but you shouldn't be surprised if they say "no."  If the seller had other payment methods listed in their ad, but will now only take a cashier's check, then you have a different problem.

When purchasing a used item from an unknown seller there is no way to receive the "protections" you receive when you purchase from an authorized dealer.  When buying used, you are incurring higher risk for the benefit of paying a lower price.  If you cannot accept the risk, then purchase from a dealer. 

Buying used speakers carries more risk than buying other gear used IMO, because the condition of speakers is more likely to be disputed due to small dings etc. that may be a big deal to some but not to others, and also because of their sensitivity to shipping damage.  Without the assurances that come with positive feedback, your perceived risk increases with respect to how the seller would handle issues such as insurance coverage, a return related to shipping damage, or other potential conflicts that go along with buying used gear.  The risk is less for the seller since they already have your money. 

Audiogon's long-time feedback process and documentation can provide greater perceived protection (that you are dealing with a trustworthy individual), and IMO (regardless of what some say) PayPal does offer some protection from fraud and goods that are not as-advertised - not perfect but better than a cashier's check.

I had the same experience. The person was a genuine first timer all was good. The second time not so good.
After that I have learned to walk away.
If you can pick them up and give them a listen ,you decide. If not hold onto your cash and wait for another pair

Don't be a Rube.  This deal stinks of a rip off.