I have 4 virtual domains bound to same account. Why isn't it an option? You can filter stuff based on "To:" address to put it in different "Inbox" folders. And storage usage is more optimal since you don't pay for a lot of nearly-empty inboxes.
If the domain is dedicated to yourself, there's no problem as you can indeed add an unlimited amount of domains, forwards and aliases. Sharing one account with 4 family members isn't very convenient though.
FastMail’s “personalities” make it possible to do this very easily. Almost all the mail clients have decent support for this too. (I use Mail.app on Mac and Dispatch on iPhone.)