In T3, you rename the title of the off canvas menu by editing tpls/blocks/off-canvas.php
What's left of blocks in T4 Blank is at local/block/mainnav.html and this file affects the content of the Mainnav section of the page, not the off-campus menu.
The offcampus functionality uses normal Joomla module management, displaying any module that is assigned to the off-campus module position. The module Title and Show Title settings are properly implemented. A search of language string constants and values did not reveal the phrase "T4 Off Canvas" so it is not fully setup to be easily renamed or translated using the Joomla language manager.
This is a non-intuitive approach. After a lengthy search, I finally found the new file that has the hard coded title "T4 Off Canvas" in \plugins\system\t4\themes\base\html\layouts\t4\layout\offcanvas.php In that code, it appears that work has been done to implement language overrides using the constant: T4_OFF_CANVAS_TITLE
<h3><?php //echo JText::_(T4_OFF_CANVAS_TITLE); ?> T4 Off Canvas</h3>
This constant was not found searching the active language strings and, unfortunately, adding this constant with a new text value in the Language Overrides did not have an effect.
So, how are normal users supposed to change the title of the Off Canvas position? Shouldn't this title be a simply a setting in the T4 Template Manager - Megamenu - Off Canvas area? A text field for entering the desired Heading for a single-language site, that gets overridden by the language override system and filled by the language system for each language if an override is found for T4_OFF_CANVAS_TITLE. This Template Manager Panel would be a good place to also set styling for this area (font sizes/color palette).
Such a visible UI attribute should be easy to set in the Template Manager, without editing PHP code, as suggested above. Until that is implemented, the file to edit should be identified in the documentation so no one else spends the hours I did trying to find this needle in the haystack.