SharePoint Office 365 public sites have built-in functionality to redesign the site and its pages.
However, if we try to edit root.master (default master page for public sites), even just adding an empty line, when customizing it, the site will throw an exception:
"The base type 'Microsoft.SharePoint.Spx.WebsiteCore.RootMaster' is not allowed for this page. The type is not registered as safe."
I tried creating a new master page with the same content, inside and outside SharePoint Designer, with no avail.
I've found that we can "overcome" this issue by setting one of the internal master pages, and replacing the main content area placeholder id from IWS_WH_CPH_Content to PlaceHolderMain.
This can also be done automatically by editing the page and selecting Style (tab) -> Attach -> pick master page.
A popup dialog will appear saying the place holder ids will be replaced as necessary and upon save the page will open with v4's look and feel.
Thanks to Amit Phule for his post