FYI: Nikon Capture 4 Product Key Utility for Windows
I opened the Nikon Message Center today to see what they're up to lately when I saw the entry for the key utility.
From what I understand it fixes your pre-v4.2 (alphabetic) key so you can upgrade to v4.2 or later. Some people were asked to enter the (now numerical) product key after upgrading to v4.2. The utility should fix that - I haven't tried it because I got my key converted by Nikon.
Read the full support article here, it also has the link to download the utility.