I have used UE 4 on a windows machine running IIS and can get the emails without issue. What is the specific environment that you are trying this on, do you know the SMTP server type and setup? Does the server require authentication?
When you get a working email using UE 3 do you specify the same settings for the mail object? If you can post back with a copy of your page that has the UE4 server behavior on it and the webassist folder from your site I can look at the pages and add some debug code to help determine how far along in the process it is getting.