Maybe it would be better if the insert was made PDF/X-4 compliant.
If you don't have control over the creation of the PDF to be Inserted, create a workflow with an Optimize Node making the insert PDF/X-4 compliant.
when trying to do a simple imposition I get error:
Document inserted into PDF/VT must be PDF/X-4 compliant.
How to remove PDF/VT info by preflight or optimize?