It can sometimes be useful to export all the people shown on a selected diagram. The following will work for V4
Create the diagram on which you want to make the split. Use expansion buttons to show only those individuals that you want to keep in the tree. You can have multiple trees in the diagram, and you canhide boxes.
Select Diagram, then Zoom, then View Whole Diagram (alternatively you can press the View Whole Diagram icon directly).
Your diagram will now be small and probably unreadable. Don't worry, you don't need to. Depress the left-hand mouse button, and, keeping it depressed, select the whole diagram.
Press Edit, then Add to Named List…
Type “Tree Export” as the name of the list. Press OK when asked if you want to create a new list.
Select File/import Export/Export/GedCom from the menu.
Click the Select button
Click on the Named List Tab
Select your new list “Tree Export”
Click » to move all the people to the right and Click OK.
Tick all the boxes for information to exclude (for ancestry or genes reunited,tick them all)
Click OK and select the filename and location you want FH to put your file.