1) When the task runs, if there is any content in areas you follow that is "new" (you haven't read it), you should get an email with a summary of that content.
2) Digests are always sent via email. The setting you referenced is your default option when you automatically follow content you are posting.
3) No question here
4) This is a change for 4.5 which we applied to our community early to facilitate mobile push notifications. In other words, your list will look like this when you upgrade to 4.5.