From what I understand (and that is not a lot!) by reading the PDF/X-4 specification on the Adobe web-site:
"All color data can be grayscale, CMYK, named spot colors, RGB, Lab or ICC-profile based."
so I am assuming that Scribus is allowing the colour schemes to be output to the PDF when in previous versions of the PDF/X standard they would be forced to CMYK. As you say, I need to force the output to CMYK while if this is not possible, convert all images to CMYK in Scribus before the export (a big job for me as the book contains hundreds of images).
Again, thanks for any help on this.