As far as I am aware, the Newsletter module is no longer supported. The best solution depends on what exactly you want to achieve, some possible options though:
If you are happy to try an untaged (“unstable”) release, then there is a SS4.0 update branch (GitHub - silverstripe-archive/silverstripe-newsletter at features/4.0-update) but this may be a work in progress.
Alternatively, if you just want to send fairly simple messages to a SilverStripe
Group you can achieve this fairly easily with some custom code (or just extracting the code you think you need from the Newsletter module). and then using a service like Mailgun or Postmark to deliver the messages.
Finally, you could use a module like https://addons.silverstripe.org/add-ons/iqnection-modules/mailchimp to push your members into MailChimp and then send emails to them using MailChimp instead.