I don't believe NCH claims they 'don't have' the old versions, It's just that they only provide downloads for the newest versions of the software and recommend that you keep a backup of the installation file with your version on it.
Right now I can only find the installation file for as far back is 4.02, I don't know if that is much help you (a license is valid for all the versions that are released within 3 months of purchasing).
I'm pretty sure I have the 4.00 installation somewhere, but I'll have to try and find it on my old back-up drives (no guarantees).
P.S. You can upgrade your license at http://nch.com.au/upgrade
If you have your old license information, depending on time of purchase etc. you'll get large discounts off the the listed prices.