Thank you for your answers.
In fact, what I was trying to do is impossible.
It's a bad knowledge of the software from my printer... who is used to Adobe, but not so much to free software.
the problem is that the PDF X-4 standard keeps the profiles of each embedded image, so if an image is RGB it will stay that way, unlike the X-1a standard which only tolerates CMYK and therefore will convert RGB images automatically.
I found the solution:
The X-4 handles transparency, but I don't need it, so I'm checking with the printer if we can just use a PDF/X-1a file instead of the X-4.
Otherwise, I want to keep the PDF/X-4, I have to convert all my images in CMYK before the integration in Scribus ...
I learned a lot about PDFs with this story !

How do you mark the subject as Resolved ?