I miss them. On the original...gulp...Ikonboard...when creating a Topic you typed in a title and could type in a subtitle and then the Post itself. Those subtitles were handy in introducing the Topic and prompted what the Post was about, above and beyond the short title. Unless I am missing something in admin area, this is not an option with IPS 4....?

It isn't a feature since IP.Board 3.0.0, IIRC. Currently, you can use a plugin:

 

Descriptions were removed in the 3.x version of the forums app @Brad Eden however the plugin above should be able to accomplish what you need.