Compatibility with Webform 7.x-4.0 branch

I will look into it, but no timeframes promised.
If someone is investigating Webform 4.x in more details, patches to make this module support it are more than welcome. Even if it's still rough around the edges.

Has anyone actually checked if it works out of the box?
If not, what are the issues that arise? Does it show the UI and configuration? Any error messages?

Yes, we intend to support it. I have done a little bit of testing and found no problems. If you encounter any problems, please open tickets for them.

I am using Webform 7.x-4.0-alpha9 and Webform Validation 7.x-1.2.

When I click on the Form Validation tab for a webform, the following log message is generated:

PDOException: SQLSTATE[42S22]: Column not found: 1054 Unknown column 'negate' in 'field list': SELECT ruleid, rulename, nid, validator, data, error_message, negate FROM {webform_validation_rule} WHERE nid = :nid ORDER BY ruleid DESC; Array ( [:nid] => 1406 ) in webform_validation_get_node_rules() (line 24 of /Library/WebServer/Documents/git/intranet-refresh/htdocs/sites/all/modules/contrib/webform_validation/webform_validation.rules.inc).

You would get this error if you had not run update.php after updating Webform Validation.