Alice, open up a Paypal case. You only have about 30 days. The seller knows this, that's why she is saying 3 - 4 weeks. She MUST provide a tracking number or proof of postage to Paypal. This will have a date on it, so you can see when the person sent it.
Sometimes a Paypal case will 'remind' them send it.
Let her know you are serious. If you receive the goods in the meantime, you can always close the case without any admonition from Paypal or Ebay. You are just keeping the seller honest, which is what Ebay is all about.
I have opened a case as I am getting really worried now. As you suggest, I can always close it if I receive the solids.
Will let you know how I go.