When you compile a 4DGL file which doesn't have a "Main" Function, a 4FN file is generated as relocatable source code. A 4FN file can be loaded, then called from the Main program that resides in the FLASH memory of the Processor.
It works just like like a function, and can be discarded when no longer required, freeing up memory. You can also pass arguments to a 4FN file from the Main Program. It can return values to the "Main" Program.
You can return from a 4FN file to the Main Program at the same point where you called the 4FN file.
Note: A good example of this technique can be found in the 4DTETRIS.4DG program which can be browsed through the Samples -> Picaso Designer -> PICASO - DEMO\EXECUTABLESSOURCE