After updgrading to DFFS v4, when you create a vLookup child item that has DFFS configured (Tabs) the vLookup In URL hidden property no longer works. If you remove the DFFS config for the new item then the hidden properties work.
The “hide in child NewForm” option was initially intended for NewForms where you did not have any other code to handle the hiding of the field. DFFS ignores this setting and will make this field visible again if the field is included in a tab or made visible by a rule.
I’ll see what I can do to make DFFS respect this setting.