Hi - stupid me. I was not entering my name exactly as originally recorded on the order. Once I figured that out, the registration entry went smoothly (thanks to your suggestion to check my SimMarket receipt!). I also (today) printed that original SimMarket receipt as a pdf file and have saved it with all my other pdf receipts (something I do religiously now with any online order), so this issue should never happen again.
Interestingly, I did in fact did copy my registration details after I ordered FSUIPC 4 (almost two years ago), but only recorded the Key and my email. When I entered my name on the recent install, I used an initial that I did not include in the original registration and that was precisely the reason why my registration was not accepted.
Thanks, Pete, for paying such good attention to this. Hope the brewery hopping was fun!