I had an issue on another site when I was upgrading to WordPress 4.0. Basically, the update went OK, but when I got to the point of Upgrading the Network to update individual sites, the upgrade had an issue. Basically, I had to deactivate and then reactivate the Domain Mapping plugin and revalidate the domains. The site was keystoliving.com
Is there a known issue with Network Upgrade and Domain Mapping, or is there any other reported issues with upgrading to 4.0 with Domain Mapping turned on? Is there a workaround, like deactivating the plugin before upgrading? I just want to know before I upgrade my main multisite (this one, opticomsites.com).