in v4, CKEditor have introduced an advanced content filter which removes unrecognized tags, see this page for details:
http://docs.ckeditor.com/#!/guide/dev_advanced_content_filter
to prevent the tag from being removed, in the webassist/plugins/ckeditor/config.js file, change:
config.enterMode = CKEDITOR.ENTER_BR;
to:
config.enterMode = CKEDITOR.ENTER_BR;
config.allowedContent=true;