When your 4D Datafile reaches close to 2GB, it is time to segment your datafile to allow for additional growth. 4D currently allows a maximum of 64 segments (the original segment, plus 63 additional segments) and each segment can grow to 2GB to allow for a 128GB maximum sized database.

In 4D 2004, you have the option to have 4D automatically add segments to your database when needed. To do this, goto 4D Preferences -> Database -> Data Management -> Create new data segment as needed

Here is a screenshot showing a 64 segmented database...