It feels inappropriate posting this on Stack Exchange, but I don't see any other viable option since the support forum has been deprecated and no-one goes there any more, so here goes:
Just upgraded to 4.6.4 on a simple up to date Wordpress site. When setting up a contribution page the Receipting tab doesn't load correctly, the clickable fields that open up into CKEditor text areas don't work. I'm seeing a JS error in the Console: "TypeError: Backbone.Marionette is undefined" and a reference to line 11 of civicrm/js/crm.designerapp.js
Can't replicate on the demo site, but then again that is apparently not running 4.6.4, but 4.6.5.