In Ghost 4 the Members feature is enabled by default, that means you don't have to do anything to enable the Newsletter form.
If you disable the Members feature, the Newsletter form will be removed automatically.
# Newsletter Page
Additionally you can also enable the Newsletter page (opens new window), to do so please follow these steps:
In the Ghost Admin head to the Pages page and press the New page button.
Give it a title (e.g. Newsletter).
You can also add a feature image in the page settings.
Publish the page.
You do not need to add content to this page as it is automatically generated.
If you decide to use another title for this page it is important that you manually change the Page URL in the page settings so that the value is
newsletter, otherwise it will not work.
If you want to use a different URL,
subscribe for example, you have to unzip the theme folder, rename the file
page-subscribe.hbs, zip the theme folder and upload it again.