Go back to HP support, and ask them 'Why' they want you to upgrade to 8.1. Personally, I am very hesitant to recommend that someone upgrade to 8.1x because of the new Licensing scheme, and without a solid reason on why to do it. Maybe they saw a fix that was only present in 8.1, I really don't know.
But, in my opinion, if upgrading takes you out of a supported version, I would not recommend that you do it
I'm pretty sure there's no good reason to upgrade at this time since we don't need the new features offered in 8.1
On the other hand, I'd like to stay up-to-date and I suspect that support for ESX 4 has been dropped only because VMWare officially stopped supporting it last month. It probably would still work under DP 8.1
Thanks for your advice. I've got the 8.1 software and new licences but I will hold on performing the upgrade