I've tried using ext-4.0.0 and it does nothing (no errors in firebug also). Tried with ext-4.0-beta3 and it works as shown in the demo.
When comparing Container.js files located in src/grid/header/, it seems several events have been renamed from 'header...' to 'column...', e.g. 'headerresize' changed to 'columnresize' (among a dozen other changes) from beta 3 to release.
Far from being an expert in Ext, I suggest to change line
(in VarHeader.js fn addListeners) to
headerCt.on('headerresize', this.handleColumnHeaderResize, this);
to make this work in 4.0.0 again.
headerCt.on('columnresize', this.handleColumnHeaderResize, this);
(having a hard time understanding Ext4)