Peer to peer host? Meaning one computer always opens the database and the other four access it with Open Remote?
Or they randomly open the database stored on one computer or server?
Databases can break and should be backed up regularly. The Recover Command in File Menu can be used to diagnose a COPY of the database exhibiting problems.
I've had users complain about lost data as well. It tends to happen more when the database is shared from a user's computer. I seems like since the host computer is taking in data from several sources, it sometimes has a hard time writing data to disk leading to data corruption or loss. Having FM Server helped to alleviate those issues.
Here are some tips.
Get your users in the habit of hitting the Enter key after entering data. Fields aren't committed until a user clicks out of the field or moves to a different record. If two users are modifying the same record, the computer shuts down unexpectedly, or loses connection with the host computer, the data might be lost. You can also include a "Commit" button on forms which will run the Commit Record/Request script.
Try to avoid entry using portals for important data entry. Clicking out of a field in a portal doesn't commit the related record; only when the displayed record is committed do the related records get committed. Instead, include a portal row button that will take them to the related record.