Hi Ruwen,
PDF/X-4:2008 and PDF/X-4:2010 differ from each other only in very minor ways. They do not differ regarding the PDF version – it is PDF 1.6 in both cases.
The fact that Adobe in earlier versions of InDesign allowed you to save a PDF/X-4 files as PDF 1.4 [and only as PDF 1.4] had the main reason, that a few years back InDesign could not yet write out layers in conformance with PDF/X-4 rules. As layers were introduced in PDF 1.5, and as it is always OK to base export to PDF/X on an earlier PDF version, they decided to resolve the lack of support for layers in an exported PDF/X-4 file by restraining PDF to version 1.4 when exporting to PDF/X-4.
Nevertheless unless you do export layers [or wanted to export images using JPEG2000 compression, or embed OpenType fonts as penType fonts...], the result is technically speaking the same ib both cases.
Now you mention that your digital print department insists on getting PDF 1.3 / PDF 1.4 – is this for really technical reasons (PDF will not at all process in the workflow) or for "cultural" reasons?
Olaf