Our problem:
Mirrorsync 4.0 is not filtering any records with the script steps in the customization script.
All data is synced ... .
We used the xml setting in the configuration utility and "Constrain found set" in the mirrorsync customization script.
The same filter worked fine in Mirrorsync 3.17
We installed the sync completely new in the database when we moved to Version 4 and copied the script steps into the new customization script.