For as far as I'm aware you need to recreate the writables from 2.x to 4.x.
Did you enable the "Enable Volumes (2.x):" in the Settings subsection of the configuration? If you select this option you can manage both 4 and 2 appstacks and writables.
Do keep in mind that updating the manager to version 4 will not work if you don;t update the agents as well. It will just use the old 2.x appstacks and writables.