I personnally tested my urls on the claim form and it worked.
If you are getting a 404 error it could be that your url is made up.
All instawallet urls that have received transactions are in the database.
I have 5 wallets, 2 with coins, 3 empty. They are definitely not made up.
I have them in my bookmarks and in a Word document.
I have tried to access using the bookmarks, and cut and paste from the Word document. Many times with no success. Over the period of over an hour.