what you say does not make me very optimistic.

Why? I just said that it's not the recommended workflow in general - but in your case, I guess it's the correct one. Just make sure you use the correct profile. (Although if you really want just four different pictures out of CMYK colors, then I guess you don't really care how those colors look like all that much... so maybe it's not needed in your case as well? If no matter the profile, you'd use 100% magenta in one place and 50% of cyan in a different place).

For the Fill layer, I figured this out. Not really that user friendly, I can admit... but in general, it looks like it's RGB by default, but just click on this little color picker button and then pick a color from palette or from the canvas (you'll see color values under the button; for canvas and the whole color selector it will be CMYK values, for other parts of Krita UI it will be RGB values - so make sure to click on something that's in CMYK!). Then just set your CMYK values as you wish.