4D Tools allows you to compact both structures and data files. Compacting consists of creating a new data or structure file that has no unused space in it. That unused space usually resulted from deleted records, in a data file, or from deleted structure elements, in the structure file.
When a database is corrupted, it is not recommended to resort to compacting since the allocation tables may be damaged and the new compacted data or structure file will reflect the contents of those tables. The rule of thumb is to always perform a repair first when you suspect your data or structure file is corrupted.