I'm posting to let you know that I have been able to verify the essence of your post. Since Amazon is usually so good I was very prepared to find that the fault lay somewhere with you. It does not. The problem, and I'm surprised that Amazon could not tell you this, is that Kindle World books are restricted to the United States. They are not supposed to be available to readers outside the US. Authors outside of the US are not eligible even to write a story in a Kindle World. I could not find this information quickly on Amazon but it is available on many of the sites for particular worlds. For example, see
http://www.carlyphillips.com/dare-to...kindle-worlds/. Or just Google it. I hope this information is out of date and KW is now worldwide, but if so I was unable to find any reference to this in my quick search.
None of the books in the series are available on amazon.com.au. It seems Amazon's problem is not that you can't purchase the 4th book. It is that you should not have been able to purchase the first three. Clearly something has changed at Amazon since your purchase of the first three books in the series. Perhaps their system previously did not enforce these restrictions in the case of a US account with an address in Australia but has since been updated. Perhaps there is some other reason.
It is not good enough. Amazon is a great company and does most things very well. But it is still often guilty of treating those outside the US and UK as second class customers. Sometimes it has little choice in the matter due to the archaic system of licensing by geographical regions. In the case of Kindle Worlds this should not usually be a problem, as authors grant Amazon the right to publish their stories on a worldwide basis. However, Hugh did sell Australian rights to Wool to one of the Big 5. Whilst I doubt any contract Hugh signed would prevent Australian distribution of Kindle World stories set in the Silo Universe, I don't think Kindle World's existed at the time, so it is a possible additional complication.
I also think a refund is far from adequate compensation when you have invested your time in reading the first three parts and are invested in reading the conclusion to the story. Barring complications from Hugh's traditional contract for Australian rights to Wool, which I think is unlikely, Amazon would have the right to sell the last book to you, making an exception to their own policy preventing it. It would of course need to be a one-off as Amazon's system obviously no longer caters for it. Your best course may well be to email Jeff Bezos directly on
jeff@amazon.com outlining the problem and asking specifically for Amazon to make arrangements for you to purchase the book.
Of course you can take steps to bypass the restriction, though you should not have to resort to this in these circumstances.
Good luck.