DP4 normally reallocates the space in the data file when records are deleted, unless you enabled 'Keep deletions' in the table details option of the database creation utility. The only reason for doing this would be to facilitate the use of the DP4 Software Release and Data Distribution (SRDD) product.
To remove deleted data from a data file with the 'Keep Deletions' flag set, run the reorganisation utility with the -REUSE command tail, as in:
REORGDB -DB SALESORD -DATA -REUSE
-REUSE is only applicable to reorganisation options 1 and 3, which rebuild the data file.