1) It'll be Forum4/000056.cgi.
If forum 4 is private, there will be a directory starting with "private-" inside Forum4. You'll find the topic file inside that private directory instead.
2) The topic format has not changed signifigantly in over five years.
After the file(s) have been restored, and permissions are set, you'll need to run a full set of maintenance (forum metadata rebuild, search index (+post history) rebuild, user subscription index rebuild).