5.1.2 is not yet released so the option left is to upgrade to 4.6.9
you can upgrade from ePO 4.6.4.
As a precaution, please make sure that you backup the ePO folders and database as per kb66616 and make sure that you go through Upgrade checklist KB71825
Can I upgrade McAfee Agent 4.6 to 4.8 before upgrading ePO? Please suggest.
Anything should be fine. You can upgrade agent either before or after upgrade.