--> Creating a new doc in 1.8.1 with CMYK/8 set as document colour
--> Placing a RGB image inside a Picture Frame Rectangle (e.g. TIFF, sRGB, 8bit)
--> Export settings PDF/X-4 ("Convert image colour spaces" not selected)
In exported PDF, the image will show as an ICC profiled RGB image if it has been neither cropped nor downsampled.
However, the image will be converted to CMYK if it has been either cropped (picture frame smaller than image) or downsampled.
I'm not sure if this is a bug or a design decision. But it would be great to be able to consistently export PDFs with embedded RGB images as it has been implemented in Publisher 1.7.3.