4D supports segmenting your data among multiple files, using the Data Segments... command in the Structure menu. This command shows a dialog that lets you add segments and to choose their sizes.
What may not be obvious from this dialog is that it is possible to resize the segments in your database. To do this, close down 4D and open your structure with 4D Tools. Within 4D Tools, choose Compact from the Utilities menu. Give the new copy of your structure a new name in the first dialog and click OK, then in the Create a data file... dialog, click on the Split... button. This will show you the same segment control dialog you can see in 4D, but with no segments showing. Now you can define a new set of segments of whatever sizes you wish (as long as they are large enough to hold the data, and remembering that they will need to be in a different place or with different names to your existing segments), and proceed to compact your old data file into the new segments.
You may wish to review the 4D Tools manual for more details about compacting data and structure files before you do this.