In July 2010 the ISO published the standard ISO 15930-7:2010 (PDF/X-4:2010):
This is a slightly revised version of ISO 15930-7:2008 from 2008, with some editorial corrections and a relaxation of the requirements for layers (optional content). The requirements of the first version were too rigid, especially for the packaging sector.
The biggest advantage of PDF/X-4 is certainly the native transparency. This means that transparencies do not have to be flattened by (poorly suited) creation applications, but can be transferred “live” into the PDF/X-4 file.
Unfortunately, however, many users are stuck in the 2000s and still use PDF/X-1a or PDF/X-3 (based on the 20-year-old PDF 1.3).
BTW: We recently approved the standard ISO 15930-9 (PDF/X-6) in the ISO commission TC130/WG2/TF2.