During the preparation of the PDFX-ready settings and recipes for the PDF/X-4 export out of QuarkXPress we have recognized that QuarkXPress 2015 and 2016 do not write a GTS_PDFXVersion entry into the PDF. (See Acrobat menu File > Properties… > Custom.)
A preflight with the PDFX-ready profiles generates an error message since we check this entry. When creating PDF/X-1a files Quark is correctly writing this information into the PDF. All other PDF/X creator I am aware do also write this entry in their PDF/X files.
This is the reason that we have decided not to publish the PDFX-ready settings and recipes for the PDF/X-4 export out of QuarkXPress yet until Quark has solved the problem.