I'm using the extension TemplateChanger.
In LibreOffice 3.6 it results in extra menu items under File > Templates.
It can import a template in an existing document.
Now in 4.0 the menu item template is canceled and is the Template dialog screen moved to File > New > Template.
I can't find the TemplateChanger options there.
I think the issue can also be solved by the maker of the TemplateChanger.
You can also Unpack that .oxt (zip) and adapt the Addons.xcu - that is the file where the menu is set.
Still, the template menu gives more problems, so leaving this issue opened for now.
I did look into the file Addons.xcu but can't find out what to change to make it work.
Other issue already for this, with some more explanation.
*** This bug has been marked as a duplicate of bug 60592 ***