The mass mailer is effectively done pending a few tweaks. There is 1 more planned sprint, and possibly 1 more after that depending on how things look. There is no official ETA, but if I were to guess I'd say beta in April.
Since 4.0 is a major release, we will likely have a couple more beta releases than normal. During beta's we usually have a release once a week. We will be looking for feedback during the beta on new functionality as well as any bugs.