Looks like as long as you have active Service and Support, you can upgrade anytime. Version 4.1 will use the current licensing model and won't be retrofitted for vRAM. Check page 15:
Q: Is there a cost saving upgrade path from V4 to V5?
A: As long as you have active Support and Subscription (SnS) there is no cost to upgrade from version 4 to version 5. If you have expired SnS then you’ll need to get that current to upgrade.
http://www.slideshare.net/softchoice/softchoice-webinar-vmware-5-release-summary
There's probably a more official source, but I couldn't find it.
EDIT: Just found it
http://www.vmware.com/files/pdf/vsphere_pricing.pdf
Q: Does the new VMware vSphere 5.0 licensing model apply
to existing licenses of VMware vSphere 4.x or prior?
A: No. The new vSphere licensing model applies only to new
purchases of vSphere licenses or to existing licenses of
vSphere 4.x or older that are upgraded to vSphere 5.0.
v 4.1 doesn't have vRAM
Q: When upgrading from existing licenses of vSphere 4.x
or older to vSphere 5.0, may I maintain the vSphere 4.x
licensing model?
A: No. In order to complete the upgrade, the new vSphere 5.
EULA must be accepted.
The EULA must be accepted when you upgrade to 5
Set-Annotation -CustomAttribute "The Impossible" -Value "Done and that makes us mighty"
