Currently, four-tilde signatures output the link to the user’s profile and talk pages and a timestamp. It could potentially be useful if other data were included in signature output, since this would allow a new discussion page interface to include more features in the rendered page without having to additionally fetch external data about users and comments. For example, if the signature output included the user ID, the interface would be able to handle renamed users correctly. If such a change is made early enough (i.e. before the full implementation of DiscussionTools), this could reduce issues with using a new discussion page interface with older discussions.
It would still be possible to replicate the old output of the four-tilde signature by typing three tildes, a space and then five tildes.
While any such change could allow more incorrect data to be displayed (that is, compared to fetching the equivalent data separately), this is already an issue with talk pages and would probably not be worsened.